EVOLUTION OF HOLASPIS 



157 



a 



Fig. 3 Dorsal views of the left posterior trunk of Holaspis. a. Skin, 

 showing double vertebral band of enlarged scales and small lateral 

 scales (only three rows shown), b. Skeleton showing elongated anterior 

 free ribs with long cartilaginous tips; the tips usually overlap each other 

 to form a continuous edge to the ribs, but in this cleared and stained 

 specimen they have become partly separated. 



posteriorly, and to a much lesser extent outwards from the vertebral 

 plates, and they also form transverse rows of which there are two or 

 three to each vertebral plate. These rows run slightly posteriorly 

 from their origin, but further out, they curve a little so that they run 

 more or less directly laterally and may turn slightly forwards before 

 they reach the perimeter of the dorsum (Fig. 3a). Each scale in a 

 transverse row strongly overlaps its neighbour on its medial flank, 

 but if the skin is pulled laterally, it stretches easily and extensively, 

 so that each scale is separated from its fellows (Fig. 4). It is then 

 sometimes apparent that the scales are interconnected by 'bridges'. 

 These are often pigmented and not very elastic and apparently 

 contain alpha-keratin, as do the scales. They lie slightly below the 

 level of the scales themselves and fall into two groups. One series 

 runs approximately laterally from the posterior outer border of each 

 scale to insert beneath the inner border of its neighbour. The other 

 runs from the anterior inner border of every scale and joins it to the 

 posterior inner border of the scale which lies in front of it when the 

 skin is unstretched. These longitudinal bridges are the only ones 

 immediately visible in preserved material, the lateral ones being 

 hidden under the imbricating scales. When the skin is not stretched, 

 the bridges are slack and slightly folded. The regions between the 

 scales and their bridges are made up of soft, extensible skin. 



Fig. 4 Scales from right lateral skin of posterior trunk, a. Skin 

 unstretched, scales overlapping medially and longitudinal bridges 

 showing, b. Skin stretched showing lateral and longitudinal bridges and 

 expanded areas between the scales. The bridge system is not always 

 fully apparent. The arrow show direction of stretching. 



presumably consisting largely of beta-keratin. The development of 

 this system of bridges shows very considerable variation among 

 specimens. 



When the skin is stretched laterally, each scale moves in a 

 transverse direction, the excursion made by the outer scales being 

 much greater than that made by the inner ones. This results in the 

 originally curved transverse rows approximating more closely to a 

 straight line. These movements can produce at least 50% increase in 

 the area of skin. This ability of the skin to expand is not so highly 

 developed as in many snakes, but it is certainly unique among the 

 Lacertidae and probably among other lizard groups. 



The dorsolateral skin, when unexpanded, has one or more longi- 

 tudinal folds on either side. The whole of the dorsal integument is 

 rather loosely attached to the underlying musculature by connective 

 tissue, as in other lacertids. The area of granular scales passes round 

 the sharp-edged lateral border of the body to contact the ventral 

 plates. These are large and arranged in six longitudinal rows, as in 

 many other lacertids, and are rectangular showing little imbrication. 

 The collar is straight-edged and again not strongly overlapping. 



Holaspis has 25-26 presacral vertebrae in males and 25-27 in 

 females. These numbers are unexceptional for lacertids in which the 

 majority of species have 25-29 presacrals with extremes of 23 and 

 33 and show sexual difference in average number of dorsal verte- 

 brae. The vertebrae of Holaspis differ from those of other lacertids 

 in being distinctly depressed with virtually no neural crest or spine 



In most lacertid lizards the dorsal ribs can be divided into three 

 groups: 1. the thoracic ribs attached to the sternum and xiphister- 

 num; 2. the anterior free dorsal ribs which are unattached distally 

 and have prominent cartilaginous extensions at their tips; and 3. the 

 posterior free dorsal ribs which are usually about two-thirds the 

 length of the more anterior ribs and have no cartilaginous exten- 

 sions. In Holaspis, there are 7-8 anterior free ribs in males and 8-9 

 in females. They are markedly elongate compared with those of 

 other lacertids, being considerably longer than the thoracic ribs and 

 about twice as long as the posterior free dorsal ones. Their 

 cartilaginous extensions are also exceptionally long and are turned 

 backwards, each extending beneath the next posterior rib and run- 

 ning parallel with its own cartilaginous process (Fig. 3b). These 

 overlapping processes are bound together by loose connective tissue 

 and form a smooth border to the series of elongated ribs. It is this 

 border which forms the prominent edge of the body that runs slightly 

 ventrally and backwards to terminate just in front of the anterior 

 border of the hind leg. The termination is enclosed in a fold of loose 

 skin that connects it to the underside of the thigh. 



The sternum of Holaspis has an extremely large central fontanelle 

 that occupies most of its area, and the scapulocoracoid plate has two 

 foramina compared with one in other lacertid lizards. 



Holaspis is peculiar among lacertid lizards in having prominent 

 slips of the intercostalis scalaris muscle (Maurer 1896) running 

 from the tips of the anterior free ribs forwards and somewhat 

 inwards to insert on the upper surface of the rectus abdominis 

 muscle above the outer edge of the second row of ventral scales. The 

 muscle fibres to the more anterior free ribs form a single block but 

 those to the more posterior ones comprise separate slips. 



Tail. In nearly all lacertid lizards, the tail is cylindrical and at 

 most slightly flattened dorsoventrally at its base. It is covered by 

 whorls of numerous subequal scales there being two whorls to each 

 caudal vertebra. Deviations from this pattern are usually slight but 

 Holaspis differs radically. Its tail (Fig. 5) is somewhat dorsoven- 

 trally compressed and above has a double row of broad plates, which 

 is a direct continuation of the series on the body. These enlarged 

 scales differ from those on the back in being arranged in simple 



