C. R. Eastman — On the Genus Peripristis. 389 



Notwithstanding the contrary opinion expressed by Jaekel/ we 

 deem it entirely proper to recognize Peripristis as the type of an inde- 

 pendent family, in view of the specialized character of its dentition. 

 As already suggested by W. Davies^ and A. S, Woodward,^ it is 

 probable, though not absolutely certain, that the dental series was 

 reduced in this genus to a single tooth in the upper and lower jaw 

 respectively. This is indicated not only by the form of the teeth, 

 but by marks of contact which show that the opposing teeth inter- 

 locked when the mouth was closed, in a manner quite inconsistent 

 with the idea that multiple series were present. The teeth are 

 always bilaterally symmetrical, none having been found which 

 correspond to the lateral series of Petalodonts. If we are to 

 suppose with Jaekel that the symmetrically formed teeth were dis- 

 posed in transverse series separated from one another by interspaces, 

 then we must needs allow that they were adjusted with almost 

 mathematical precision, and that there could have been not the least 

 lateral play of the jaws in closing. Otherwise it would have been 

 impossible for the functional rows of teeth in opposite jaws to have 

 fitted as accurately into one another as is known to have been the 

 case, the median acuminate apex of the lower teeth being received 

 into a corresponding pit of the upper. So complicated a device 

 as this theory calls for is unparalleled throughout the whole animal 

 kingdom, and sets ordinary mechanical conditions at defiance. It is 

 possible that our German friend has not taken considerations of 

 this nature into account, or he would scarcely have hazarded the 

 opinion " dass Pristodus mehr Querreihen als alle Petalodonten im 

 Gebiss besass, und in deren schwacher Verfalzung die Entwickelung 



der Petalodonten einleitete Wenn wir uns die Zahne 



von Pristodus in derselben Weise auf langere Kieferaste vertheilt 

 denken, wie die von Orodus oder Cladodus, so hinderte nichts eine 

 ungetahr symmetrische Ausbildung der einzelnen Zahne." * 



It is the object of the following paragraphs to present a few new 

 facts concerning the structure and distribution of teeth belonging to 

 two species of this genus, namely, P. seinicircularis and F. coiicinnus. 



Family PERIPEISTID^, nomen nov. 



Genus Peripristis, St. John. 



[Proc. Amer. Phil. Soc, vol. xi (1870), p. 434.] 



Syn. Hoplodus, R. Etheridge, jun. : Geol. Mag. [2], vol. ii (1875), p. 244. 

 Biodo-ntopsodiis, J. W. Davis : Brit. Assoc. Rept., 1881, p.. 646. 

 Fristodus, J. W. Davis (ex Agassiz MS.) : Trans. Roy. Dublin Soc. [2], vol. i 

 (1883), p. 519. 



Peripristis seinicircularis (Newb. & W.). 



1866. Ctenoptychius semicircularis, Newberry & AVortlien : Pal. Illinois, vol. ii, 

 p. 72, pi. iv, %. 18. 



.1 Zeitschr. dautsch. Geol. Ges., vol. li (1899), p. 290. 



2 Geol. Mag. [2], vol. ii (1875), p. 243. 



3 Cat. Foss. Fishes Brit. Museum, pt. i (1839), p. 62. 

 ■* Log. cit., p. 292. 



