FOSSIL FISHES, 609 



at the same time the most characteristic are the lower jaws or mandibles. 

 It is consequently advisable for the present purposes to define the various 

 species by these organs, reserving the special appropriation of the other 

 plates and bones to their several jaws until accidental discovery of speci- 

 mens showing them in direct or indirect combination shall furnish the 

 requisite evidence, or until their structure can be interpreted by the anal- - 

 6gy of kindred or similar species. By this plan the multiplication of 

 names is avoided. 



On this method of distinction we now add to the genus a third 

 species whose jaw differs from the other two in the absence of curvature. 

 Wide at the spatulate or hinder end it narrows and thickens forward 

 developing the gouge-like form of the other species by the incoming of 

 the arweolus. In consequence of this straightness which at present suf- 

 ficiently characterizes the mandible, we propose for the species the name 

 of T. rectus. 



One of the most remarkable of the fish fossils of the Cleveland shale 

 lately discovered is figured on Plates XXXVIII and XXXIX. It consists 

 of three bones, two at least of which are in their natural position. The 

 largest of the three — an exceedingly massive plate — measures 17 inches 

 by 17 and is of the form of a carpenter's square, consisting of two arms 

 meeting at a rounded r.ght angle. Of these two arms one, as shown, is 

 a plate perfectly fiat on one face and straight on one (outer) edge while 

 the other, curving from the point sweeps out and meets it at a slightly 

 acute angle. Both edges are thin, the latter showing a slight underlap- 

 ping margin. The other arm, also nearly plane, is narrow and much 

 thicker than the former and makes at its end a sutural connection with 

 the bone next to be described. 



From the extreme point of the first plate there rises gradually a 

 strong flange which becomes higher and thicker as it nears the angle. 

 At length it separates and forms a wide, strong bone nearly as heavy as 

 the other which it really doubles. (PI. XXXVIII.) This also ends in a 

 form that seems to indicate sutural connection with some other bone. 

 Considerable space is left between the branch and the main plate at their 

 ends but a curved bony connecting plate exists throughout. 



To the suture first mentioned at the end of the plate is firmly united 

 the second of the series, which is of the same general form as the pre- 

 ceding but its angle is turned in the opposite direction. This bone is 

 very thick and rounded. One arm measures five inches and the other 

 four inches in length. Its free end indicates a suture to form a connec- 

 tion with another yet unknown. 



The third bone of this set lay across the second in its angle. Its 

 length is about ten inches and it is somewhat club-shaped, tapering down 

 from the large end which is in contact with the other bone. See figs, 

 plates XXXVIII and XXXIX. 



39 G. O. 



