146 



SUB-CLASS ELASMOBRANCHII. 



five gill-arches but there may have been more. Two dorsal fins have 

 been seen (one only shown in Fig. 84), but no anal. The caudal fin 

 is strongly heterocercal ; the neural arches are continued to the end 

 of the tail, and carry stout somactids which extend to the edge of the 

 fin. The paired fins are horizontal expansions of the integument. 

 The peripheral somactids are parallel, unsegmented, and extend to the 

 margin ; between their distal ends are slender cartilages which are pos- 

 sibly displaced somactido. Thebasals are also parallel, and are contained 



FIG. 84. Restoration of Cladoselache newberryi Dean (from Woodward, after Dean). 



ii 



in the body wall. The skin is covered by minute denticles, not enam- 

 elled. Cladoselache Dean, Lower Carboniferous of Ohio ; Cladodus Ag. 

 for some tune known only by teeth ; Devonian, Carboniferous, and Per- 

 mian. 



Order 2. ACANTHODII.* 



With dermal calcareous plates on the skull and pectoral arch, and with 

 a mosaic of quadratic dermal scales on the body. All the fins except the 

 caudal, with a powerful dentine spine on their anterior margin. Without 

 claspers. There are no cranial bones, nor membrane bones connecting the 

 pectoral arch with the cranium. 



This group, which was formerly placed with the Ganoids, is now placed 

 with the Elasmobranchs. The endoskeleton contains granular calcifica- 

 tions, and the dermal plates placed on the head, body and pectoral girdle 



FIG. 85. Acanthodes Wardi x i (after Woodward), 

 gill-frills are hypothetical. 



The orbit is made too small and the 



seem to have consisted of vaso-dentine or of structureless lamellae without 

 bone-cells. The most marked characteristic of the group is the large 



* Huxley, Geological Survey of the United Kingdom, 10, 1861. Fritsch, 

 Fauna der Gaskohle in Bohmen, 2, 1889. Reis, Zur Kenntniss des Skelets 

 der Acanthodinen, Geognost. Jahreshefte,'Munchen, 1890, 1894. Traquair, 

 Geol. Mag., 1888, p. 511 ; 1889, p. 17. 



