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The basic anatomy of the lacertid hemipenis (intromittent organ) and methods for its investigation are described. In many members of the Lacertidae, the hemipenis has a structure quite unlike that of other squamate reptiles: the distal lobes of the retracted organ are complexly folded and there is a well-defined supporting structure of dense connective tissue, the armature. This incorporates blood sinuses and has an intramuscular portion embedded in the m. retractor penis magnus and two club-shaped bodies, the clavulae, that support the lobes in the erect organ. Unarmatured hemipenes occur in some lacertids and, like those of other squamates, possess sac-like lobes in the retracted state, but they are singular in having the lobes invested by the m. retractor penis magnus. It is argued that many of these apparently primitive hemipenes are in fact secondary derivatives of the armatured type. There is considerable inter-specific variation in hemipenial structure which is described systematically. In some cases this involves differences in size, asymmetry and simplification, which may arise as physical isolating mechanisms and is useful in distinguishing otherwise very similar species, particularly in the genus Mesalina (p. 1253). Other shared derived hemipenial features provide useful information about relationships between species and higher taxa and a summary of the hypotheses that they support is given (p. 1254).

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Latastia taylori spec. nov. (fig. 4) Latastia boscai (part) Parker, 1932, Proc. Zool. Soc. London, p. 355 (specs, e-m). Holotype a male, number 1931.7.20.337, in the British Museum, from the Buran Valley, 2500 ft. (10°20`N x 49°E); collected by Capt. R. H. R. Taylor, 17.X.1929. Head flat, depressed, once and three quarters as long as broad, its depth a little less than the distance between the tip of the snout and the anterior corner of the eye, and its length contained 4.25 times in the length from snout to vent. Nostril pierced between four shields; upper nasals forming a suture half the length of the fronto-nasal which is a little broader than long and broader than the internarial space; prefrontals forming a median suture shorter than that between the nasals; frontal not grooved, a little longer than its distance from the rostral, once and two thirds as long as broad; interparietal not quite twice as long as broad, in contact with an occipital half its length; 4 supraoculars, the first divided into two, second and third large and subequal, fourth very small; a row of granules separating the supra- oculars from the 5 supraciliaries. Lower eyelid scaly, translucent. Rostral not entering the nostril ; two superposed post-nasals, the lower in contact with the first and second labials ; anterior loreal half as long as the second; five or six labials anterior to the subocular, which is much narrowed on the lip and separated by two scales from the pos- terior loreal ; lateral edge of the parietal bordered by 3 elongate, narrow scales, of which the anterior is much the longest; anterior margin of the ear bordered by 3 or 4 scales of which the uppermost is the largest. Four pairs of chin-shields, the first 3 in contact and the last the largest; 31 gular scales between the chin-shields and the collar which has about nine scales on its edge, the median very large and the laterals grading into the granules of the neck. Dorsal scales oval, or subhexagonal, flat and smooth, in 39 series across the middle of the body and in 105 series between the occipital and the base of the tail (vertically over the vent) ; twenty-two in a transverse series between the hind limbs. Ven- trals in six longitudinal series, with straight posterior borders, the two median series much narrower than the others; no group of small pec- torals; twenty-five transverse series of ventrals; one very large preanal bordering the vent, preceded by another, but much smaller, shield. A series of enlarged plates beneath the fore-arm; upper tibial scales small, imbricate, keeled; ten or eleven femoral pores on each side; sub- digital lamellae strongly bicarinate 26 beneath the fourth toe. Caudal scales in equal whorls, oblique and strongly keeled above, smooth be- neath. Tip of the fourth toe reaching to midway between the arm and the ear. Pale reddish brown above and on the tail, faintly marbled with grey anteriorly and on the head; flanks anteriorly and side of the neck with very irregular brown and greyish-white vertical marblings. Lower sur- faces uniform white. Length from snout to vent 43 mm. Fore-limb 13 mm. Hind-limb 26 mm. Tail (regenerated in part) 87 mm. The following specimens are paratypes of this species: 1931.7.20.339 9 2000 ft. 10°15`N x 49°E 13.1.1930 1931.7.20.338 9 20 ft. 11°14`N x 49°E 3.XII.1929 1931.7.20.340-342 3 & & 2000 ft. 9°40`N x 49°E [Ex. Field Mus.j 8.II.1930 19317.20.335-336 d 9 3100-3300 ft. 10°13`N x 48°46`E 8.1.1930 This series shows the following variations from the holotype: The head may be once and two thirds as long as broad, and its depth equal to the distance from the snout to the eye; supraciliaries 5 to 7; one or two scales between the posterior loreal and the subocular; 5 or 6 labials anterior to the subocular; dorsals in 36 to 41 series across the middle of the body; ventrals in 23 to 26 transverse series; gular scales 28 to 32; plates in the collar 5 to 7; femoral pores 9 to 12; subdigital lamellae beneath the fourth toes 24 to 27. The fourth toe extends to some point between the shoulder and the middle of the neck. The colour is usually olive, almost uniform, but with traces of lighter marblings anteriorly and on the sides of the neck and anterior part of the flanks, the latter having a tendency towards the formation of verti- cal bars. The subcaudal scales are smooth proximally, but keeled dis- tally and an unregenerated tail is a little more than twice as long as the distance from snout to vent. These specimens were originally believed to be all immature, but a female of 42 mm. from snout to vent is pregnant and the species ap- pears to be consistently smaller than boscai. It is closely allied to the latter but may be distinguished by its broader, natter dorsal scales, the absence of a frontal groove and different colour; it appears to be restricted to the north-eastern districts of Somaliland from the Sol Haud to the coast, an area close to that in which the strongly striped L. boscai burii also occurs (fig. 4).