J. W. DAVIS ON THE GENUS PLETJEICANTHIIS, AGASS. 



333 



Dr. Goldfuss, in 1847, described and figured Orthacanthus J)e~ 

 cTienii*, found in Coal-measure sandstones at Ruppersdorf, in Bohe- 

 mia. The whole remains of a fossil fish were found, with the spine 

 still attached to the back part of the head. The fish probably mea- 

 sured 18 inches, and the spine 4-8 inches. The spine is described as 

 " a single, straight, plain, circular, bodkin- or spike-shaped spine, 

 "bearing on the posterior surface a row of barbs or denticles slightly 

 distant, alternately right and left of a somewhat elevated median 

 ridge." An enlarged figure of the spine is also given. The distinct 

 median keel, together with the alternate and widely separated den- 

 ticles, appear to distinguish this spine sufficiently from any other 

 species since described. Its nearest relative is 0. cylindricus of 

 Agassiz, with which it agrees in being straight, circular, and taper- 

 ing to a point. 0. cylindricus, however, has no median keel ; and 

 its denticles are close and, for the most part, opposite. 



Orthacantlius hohemicus, Pritsch, from Kounova, Bohemia, appears 

 in most respects to be closely related, if not identical with, P. cylin- 

 dricus. It is straight, and finely striated longitudinally. The two 

 TOWS of denticles are rather closely approximated, and appear to be 

 separated by a median groove. 



The three species P. rohustus, P. alternidentatus, and P. dlatus are 

 extremely interesting, because they serve to bring together into one 

 genus the Pleur acanthus and Orthacantlius of M. Agassiz. They form 

 intermediate links between the two ; the lateral teeth of PUura- 

 canthus Icevissimus give place in these species to others, which, though 

 not so widely placed as in P. Icevissimus, are still very wide apart ; the 

 posterior surface enclosed by the two rows of teeth occupies fully one 

 third the circumference ; and in this respect they difier equally from 

 the Orthacanthus cylinchncus figured by M. Agassiz, in which the 

 two rows of denticles are very close together. 



Orthaatnthiis is a circular spine. Not only does this apply to 

 0. cylindricus, but to other species which have since been described. 

 Pleuracanthus is depressed, forming an oval section. In the species 

 under consideration a triangular form is assumed, caused by the rows 

 of denticles projecting beyond the basal line at its junction with the 

 sides ; this is especially the case in P. alatus. It has already been 

 mentioned that the specimens of P. (Orthacanthus, Ag.) cylindricus 

 vary much with respect to the position of the posterior denticles ; 

 in the type specimens they are quite near together, whilst others, 

 similar in all else, have the rows of teeth placed wide apart. In the 

 example figured (fig. S a) the denticles are almost as widely sepa- 

 rated as in P. rohustus or P. cdatus. Taking all these circumstances 

 together, we are driven to the conclusion that there is no generic 

 difference between Pleuracanthus and Orthacanthus. When Prof. 

 Agassiz described the two genera, only the extreme forms were 

 known ; and they appeared sufficiently distinct to warrant their generic 

 separation. During later years many other intermediate forms have 

 been found ; and, as was suggested by Sir P. Egerton, the approxi- 



* Eeitriige zur Yorwelthcben Fauna des Steinkohlengebirges (Bonn, 1847), 

 plate 5. Hgs. 9-11. 



