90 



VERTEBRATE SKELETON 



genus differs from other Notidanids in not being amphistylic, the 

 otic process not reaching the cranium. Pliotrema (So. Africa, near 

 Acanthias) differs from its relatives in having six branchial arches. 

 A few fossil genera of Squali {Edestus, Helicoprion) have the lower 

 jaw greatly elongate and spirally coiled, the coils armed with large 

 teeth of problematic use. 



Few details are known of several groups of fossil Elasmobranchs. Three of 

 them (Ichthyotomi, Cladoselachii and Acanthodii) are amphistyHc like Noti- 

 danids. Most have five gill-clefts with corresponding arches (some Clado- 

 selachii may have had six or seven) which- are divided into four parts, the 

 pharyngobranchial possibly being absent. The rostrum is small and the mouth 

 nearly terminal. No bone is developed, but the cartilage was sometimes 

 calcified. 



OSTRACODERMA.— The oldest and least understood Vertebrates grouped 

 under this head had an anterior region ('head') which in the simplest forms 



Fig. '94. Fig. 95. 



Fig. 94. — Dorsal and ventral restorations of Bothriolepis canadensis (Traquair, '04). 

 adl, antero-dorsolateral plate; ag, angular; amd, anterior median dorsal; avl, antero- 

 ventrolateral; c, centrals of lower appendage; da, dorsal anconeal; dar, dorsal articular; 

 el, extralateral; em, external marginal; im, internal marginal ; /, lateral; /o, lateral occip- 

 ital; m, median; mi, marginals of lower appendage; mo, median occipital; mv. median 

 ventral; mx, maxillary plate; o, ocular; pdl, posterior dorsolateral; pm, premedian; 

 pmd, posterior mediodorsal; pvl, posterior ventrolateral; si, semilunar; va, ventral anco- 

 neal; var, ventral articular. 



Fig. 95. — Dorsal side of head of Pteraspis (Lankester, '68). 



{Lanarkia, Thelodus) is covered by numerous irregularly arranged scales, 

 apparently placoid. Others have the scales united in plates (not true bone) 

 and others the whole head is enveloped in such an armor (figs. 94, 95). It is 

 impossible, at present, satisfactorily to homologise these plates with the bones of 

 the normal ossified skull, but there is an approach to the cranium of higher 



