ICHTHTOTOMI. 147 



spines on the front of the fins. These appear to have consisted of dentine, 

 and are doubtless comparable to the spines found in similar positions in 

 the fins of Elasmobranchs. It is probable that a number of isolated 

 spines which have received special generic names (Onchus, Byssacanthus, 

 Homacanthus, etc.) may have belonged to fishes of this group, and that 

 a number of quadratic scales, e.g. Thelolepis, Coelolepis, etc., from the 

 Upper Silurian, were part of the dermal armature of similar fishes. Pec- 

 toral and pelvic fins are always found. 



Fia. 86. — Climatius scuHger, outline of fish with spines shaded. The pectoral fins pines pet 

 are the two large spines next the head ; then follows a double row of smaller spines i, the 

 last of which are the pelvic fin-spines plv. The large fln with spine a between the paired 

 spines and the caudal flu is the anal ; d\ d^ dorsal fins. 



The eye is surrounded by dermal plates ; the notochord must have been 

 persistent ; the supports of the fins are not preserved ; the tail is hetero- 

 cercal and the caudal fin without any trace of upper lobe. Comparatively 

 small fishes. Acanthodea Ag. (Fig. 85), Lower Devonian to Lower 

 Permian ; Diplacanthus Ag., and Climatius Ag., Upper Silurian and 

 Lower Devonian, without teeth, with four or five pairs of spines between 

 the pectoral and pelvic fins (Fig. 86). Most of the sub-order do not 

 show teeth, but there is a powerful dental armature in lachnacanthua 



Order 3. Ichthyotomi.* 



The cartilaginous endoskeleton is permeated by granular calcifications ; 

 notochord unconstricted, with slight calcifications in its sheath ; neural and 

 haemal arches calcified, with long spinous processes ; tail diphycercal ; 

 pectoral and pelvic fins with long segmented axis and biserial radii ; pelvic 

 fins with claspers in the male. The teeth have two large lateral cusps, with 

 one small median cusp. Lower Carboniferous to Lower Permian. 



Pleuracanthus Ag. {Xenacanthua Beyr.) (Fig. 87). Body elongated, 

 to half a metre ; skin probably naked, with a long spine attached to the 

 occipital region of the cranium ; five, possibly seven, branchial arches ; 

 all the fins with dermotrichia ; pectoral girdle arch-Uke, united with its 

 fellow, composed of two pieces ; pectoral fin (Fig. 76) with segmented 

 axis ; rachiostichous and pleurorachic, or nearly so (p. 57) ; pelvic fin 

 similar, but with no postaxial somactids, with claspers ; pelvic girdle of 

 two separate arches. The somactids of the long dorsal fin are segmented 



* Fritsch, loc. cit. Davis, On the Fossil Fish Remains of the CoEd 

 Measures in the Brit. Islands, I. Pleuracanthidae, Trans. Roy. Dublin 

 Soc, 4, 1892. 



