THE PERMIAN FISHES OF NORTH AMERICA I57 



This species is perhaps identical with Janassa strigilina, but based on 

 a lateral tooth, whereas the former is based on a median one. In the 

 absence of evidence on this point, however, it is best to retain the name 

 strigilina, at least provisionally. 



Original description: "The tooth is quite small, its length only equaling 

 the width of the known tooth of S. [Janassd\ linguaformis. It is also nar- 

 rower in proportion to the length. The root and the cutting edge are turned 

 in opposite directions, as in the other species. The principal difference 

 between the two is seen in the character of the transverse ridges or crests 

 of the oral face. There are two crests less, or five, with a delicate basal 

 fold, making six, while, counting the fold, there are eight in S. \Janassd\ 

 linguceformis. The anterior ridge is transverse; the others slightly convex 

 backwards, and all are equidistant and uninterrupted, which is not the case 

 in the older species. They are also of different form, being distinct ridges 

 with anterior and posterior faces similar. In S. [Janassa] linguaformis the 

 anterior face only is vertical, the posterior descending very gradually, the 

 whole forming a series of steps. 



"Length of ridged face, 0.0060 m.; width anteriorly, 0.0035 "^-J width 

 posteriorly, 0.0020 m." 



Janassa ordiana Cope (Am. Nat., xv, 1881, p. 163, and Trans. Am. Philos. 

 Soc, XVI, 1888, p. 285) was mentioned by name only but never described. 

 Its inclusion in the list was probably due to an oversight on Cope's part, and 

 it should be dropped. 



Thoracodus emydinus Cope (Proc. Academy Nat. Sci., Phila., 1883, 

 p. 108) is, as suggested by A. S. Woodward (Catalogue Fossil Fishes British 

 Mus., I, p. 39), an incomplete Janassa tooth; but it is too imperfectly 

 defined to stand as a valid species. 



(?) Genus HYBODUS Agassiz. 



There are in the American Museum collections a number of fragments 

 of a spine (No. 7263), too imperfect for description, which indicate, appar- 

 ently, a species of Hybodus (plate 30, figs. 5, 5^). The fragments represent 

 a large spine ornamented with smooth, coarse ridges closely apposed to one 

 another and with a single series of small denticles running down the middle 

 of the posterior face. Anterior margin of " cut water " without an enlarged rib. 



ICHTHYOTOMI. 



Genus PLEURACANTHUS Agassiz. 



Poiss. Fos.j III, 1837, p. 66. 

 The type genus of the Ichthyotomi — a group of extinct, primitive 

 sharks ranging from the Upper Devonian to the Triassic, and reaching 

 its maximum evolution in the Carboniferous. 



Revised description of genus: 



1. Skeleton cartilaginous, with minute prismatic calcifications. 



2. Notochord persistent. 



3. Neural and hsemal arches present. 



4. Paired fins of the archipterygial type with segmented lateral 



branches. 



5. Dorsal fin without spine, low, extending from a little back of 



the occiput to origin of caudal. 



