250 NATURAL SCIENCE. April, 



membrane bones, is studded with minute lozenge-shaped denticles- 

 No enamel appears to be present on the denticles of the body-region, 

 and in a recent paper of Professor Claypole (Proc. Amer. Micr. Soc, 

 pp. 192-195; 1894) i* * s shown to be also absent on the teeth. The 

 Cladodont teeth are arranged on each mandibular ramus in a dozen 

 or more banks, each composed of about seven teeth. Of these the 

 innermost are the largest, and of all the banks those most nearly 

 symphysial include the stoutest and longest teeth. At the hinder 

 mouth-margin, the shagreen denticles, here enlarged, approximate in 

 size and shape to the smallest of the teeth (as in Chlamydoselache): they 

 are also notably present in the enlarged shagreen-plates surround- 

 ing the orbits. 



(d.) Fins and Girdles. — The paired fins of Cladoselache must be 

 looked upon as lappet-shaped remnants of the continuous dermal fold, 

 which is generally believed to be the ancestral condition in the history 

 of the paired limbs. And it is certainly most interesting evidence 

 in support of the lateral fold theory to find that in this most 

 ancient of typical sharks the fin-conditions are precisely what one 

 might have expected on purely a pviovi grounds. The pectorals and 

 ventrals are entirely lacking in lobate bases. The fins lie in the plane 

 of the fish's movement, the hinder limits of the ventrals drawing 

 closely together in the region of the anus ; their external supporting 

 elements, the radials, extend metamerally from the body-wall to 

 the fin-margin, and there is practically no dermal margin surrounding 

 the fin. 1 The pectorals are clearly the more specialised : they are 

 enlarged in size, their anterior supporting elements, thickened and 

 blunted, forming a compact cut-water, and the elements of the fin 

 include the compacted structures 2 of between thirty and forty meta- 

 meres. The ventrals, on the other hand, retain, as Wiedersheim's 

 work leads us naturally to expect, the more primitive characters ; 

 although they present a smaller number of metameral supports — 

 twenty-two to thirty — they are but the elongated remains of the 

 dermal fin-fold, whose margin is slightly blunted anteriorly, where the 

 supporting elements have become compressed into a serviceable cut- 

 water. In the mid-region of these fins, as also in the pectorals, it is 

 doubtless the compression of the radials which has caused their tips 

 to intercalate, as shown in the figures. 



As to the supports of the paired fins, in the ventrals (Fig. 5) the 

 basal cartilages, B, are as yet segmental and unfused, and no form of 

 pelvic girdle appears to be present. The supporting elements of the 

 pectorals are not as yet to be made out clearly ; their probable dis- 



1 This condition is nearly parallelled in the recent bottom-living Elasmobranchs ; 

 in these, however, the radials are invariably jointed, and their fins, great in size, are 

 not directed downward, as the fossils indicate, to the plane of the pectorals of 

 Cladoselache. 



2 The present writer has preparations which show that the intercalated radials 

 traverse the entire fin ; it is accordingly evident to him that these rays may justly 

 be regarded in all cases as homologous structures 



