940 CLASS PISCES. 



gular shape, with the outer border slightly involuted, and an undu- 

 lating coronal contour in the upper jaw ; the genus occurring in 

 Europe and the United States. Other genera which can be merely 

 mentioned are Tomodus, Xystrodus, Deltodus, and Pcecilodus ; the 

 three last being common to Europe and North America. Teeth 

 referred to Poerilodus, and to the above-mentioned genus Psephodus, 

 have also been described from the Carboniferous of Northern India ; 

 and the teeth from the same beds described under the name of 



Helodopsis are likewise referable to 

 the present family. In the type 

 genus Cochliodus, of Europe and 

 the United States, there are two 

 pairs of dental plates in the man- 

 dible (fig. 863), in which the outer 

 border is much involuted. The 

 posterior plate is elongated antero- 

 posteriorly, and has its anterior 

 Fig. 863.— Lower dental plates of CoMi- and posterior borders converging 



odus contortus ; from the Carboniferous of , ji j • , t_t 



Armagh. One-half natural size. The sped- outwardly, and a prominent oblique 



men is viewed from the anterior aspect, and -r\Acrc± liL-Awic^ normminrr in flip 

 the white line on the left side shows the divi- nCl & e ' ^WlSe narrowing in ttie 



sion of the two plates. same direction, in the middle of 



the crown. The anterior plate is 

 narrow from before backwards, with a ridge on the hinder border of 

 its crown resembling the median ridge of the larger plate. The 

 remaining genera of this family are Streblodus, Deltoptychius, Diplac- 

 odus, and Cyrtonodus, of which the two last are doubtfully entitled 

 to distinction ; while other names have been applied to detached 

 anterior teeth. 



Family Cestraciontid^e. — This family, which may be taken to 

 include the Orodontidce. and Hybodontidcz of many writers, is repre- 

 sented by a considerable number of genera ranging from the Car- 

 boniferous to the present day; all the existing species being included 

 in the type genus Cestracion, of which the upper dentition is shown 

 in fig. 864. The family is characterised by the presence of a spine 

 to each dorsal fin, of which the first is placed immediately above 

 the interval between the pectoral and pelvic fins. The teeth are 

 generally more or less obtuse, with several series in use at the same 

 time, and those of each oblique series never fused into continuous 

 plates. One of the oldest genera is Orodus (more correctly Oreodus), 

 of the Carboniferous of Europe and the United States, in which the 

 teeth are of the general type of those oiHybodus, and are only regarded 

 by Mr S. Woodward as entitled to separation on account of the 

 absence of other associated remains which are. characteristic of the 

 Mesozoic genus. It is probable that some of the dorsal fin-spines 

 originally described under the name of Ctenacanthus belong to 



