76 PISCES. 



fused together and with the premaxillse (p.mx.}. The cheek is entirely 

 covered with loose plates, of which the suborbitals (s. o.) are conspicuous; 

 the dentigerous maxilla (mx.} bounds these below, and exhibits a small, 

 apparently jugal plate (%'.} behind its posterior expansion. The latter 

 element extends far backwards, and immediately above it is a very large 

 cheek-plate (x.) covering the whole of the space between the posterior sub- 

 orbitals, the cranial roof, and the preoperculum. At the base of the skull 

 there is a well-ossified parasphenoid, meeting in front a pair of vomers, 

 each of which bears a powerful tooth ; and there are some traces of an 

 inward palatal extension both of the maxillae and premaxillee. The man- 

 dible exhibits the usual complexity and seems to possess a distinctly 

 ossified articular element. The dentary bone (d) is relatively deep and 

 thick at the symphysis, tapering backwards, and bears a series of small 

 teeth, with a single large cutting tooth (laniary) in front. Its lower border 

 is bounded by a series of four plate-like, lenticular bones, of which the 

 hindermost seems to correspond to the angular (ag.), while the others are 

 conveniently termed infradentaries (i. d.). A thin splenial lamina forms 

 the inner wall of the ramus, while between this and the dentary is 

 arranged a series of about three very stout lenticular bones, each of 

 which bears a large tooth. A deep and narrow preoperculum (p. op.) is 

 conspicuous, while the operculum (op.) and suboperculum (s. op.) are well 

 developed, without any trace of an interoperculum. Below the subopercu- 

 lum a long narrow plate forms the hinder element of the series of lateral 

 gulars (/. j.) on each side; and a pair of very large principal gular plates 

 (_/.), with a small anterior azygous element (m.j.), occupies the whole of 

 the space between these series. The teeth are round in section. The 

 cranial roof is bordered behind by three small supratemporal plates (s. t.), 

 one median and a pair lateral ; but there appear to be no large scales on 

 the posterior margin of the pectoral arch. There are well-ossified ring- 

 vertebrae. The infraclavicle is destitute of an ascending articular process ; 

 the two dorsal fins are remote and directly opposed to the pelvic and anal 

 fins ; the tail is heterocercal, the caudal fin rhombic in form. The scales 

 are thin, ovoid, and deeply imbricating, each marked with a median boss 

 on its inner face. The typical species R. sauroides, attaining a length of 

 not more than 0'5 m., is widely distributed in the Coal Measures. 



Eusthenopteron (figs. 23, 58). A fish closely resembling Rhizodopsis, 

 but with laterally compressed teeth and nearly diphycercal caudal fin, 

 which is truncated or wavy at the hinder border. A pineal foramen 

 occurs between the frontal bones. The eye is provided with a ring of 

 sclerotic plates. The examples of the typical and only known species, 

 E. foordi, from the Upper Devonian of Canada, are important as display- 

 ing the calcified supporting cartilages of some of the fins. The pectoral 

 fin (figs. 23, 58) has only one stout basal cartilage, and is simply Gegen- 

 baur's " archipterygium " partly atrophied, with some elements fused 

 together. The bases of the second dorsal and anal fins are shown to be 



