i8o HOLOCEPHALI 



BO characteristic of the dorsal fin of the Holocephali. A better knowledge 

 of the histological structure of the dermal plates of these families is much 

 needed. 



Ptyctodus, Pander, Bhynchodus, Newb. ; Devonian, Europe and N. 

 America. Palaeomylus, A. S. W. ; Devonian and Carboniferous, N. 

 America. Rhamphodus, Jaekel ; Devonian, Germany. 



Family MENASPIDAE. In this family may be provisionally included 

 certain incompletely known fossil fish the affinities of which are still very 

 uncertain, but which appear to be more closely related to the Holo- 

 cephali than to any other order. 



Menaspis has a general covering of denticles strengthened into a sort 

 of cephalic shield bearing a series of paired lateral spiny processes, per- 

 haps homologous with the spiny plates of Myriacanthus. Three unequal 

 pairs of long curved rods of calcified fibrocartilage may be compared to 

 the so-called labial cartilages of the same genus. The dentition consists 

 of four grinding plates like those of Rhynchodus. Such remains of the 

 paired fins and mucus canals as have been found resemble those of 

 Chimaerids. Menaspis seems to be allied to Myriacanthus (Dean [1106]). 

 It is possible that the Devonian fossil described by Traquair under the 

 name Gemundina Sturtzii [467] belongs to this group. 



Menaspis, Ewald ; Permian, Europe. 



Sub-Class 2. PLEURACANTHODII (Ichthyotomi). 



Of all the Palaeozoic Chondrichthyes the Pleuracanthodii are 

 by far the best known, their skeleton having been described in full 

 by Brongniart [62], Koken [269], Doderlein [113], Reis [353], and 

 others, and especially in the admirable work of Fritsch [139]. 



Pleuracanthus is shark-like in shape, with a large nearly terminal 

 mouth, a powerful median occipital spine, well-developed paired 

 fins, a long tapering tail, almost if not quite diphycercal, and 

 elongated median fin-folds. The dorsal fin is almost continuous, 

 begins but a short way behind the head, and is only separated by 

 a notch from the caudal. The median ventral fin is subdivided 

 into two anal lobes and a long caudal (Fig. 146). 



The dermal skeleton is little developed, being represented by 

 small denticles near the mouth, along the back, and on the clasper 

 of the male. Large pointed tricuspid teeth arm the jaws ; like the 

 occipital spine they are formed of vasodentine (Fig. 148). 



The endoskeleton was entirely cartilaginous, with well- 

 developed prismatic calcifications. The skull and jaws resemble 

 those of the Notidanidae ; the pre- and postorbital processes are 

 large, and the attachment of the jaws is of the amphistylic type. 

 Five branchial arches, subdivided like those of the Selachii, are 

 present (bearing small teeth) ; the median basibranchials seem to 

 be somewhat reduced in number. The notochord was persistent, 

 and probably quite unconstricted ; no distinct centra are found 



