1884.] ^oJ [Cope. 



(4.) The identity of Didymodus (type Diplodus compressus Newberry) 

 and Pleuracanthus, may now be questioned. None of the specimens are 

 figured and described by the authors above cited, as displaying an occi 

 pital condyle, or posterior frontal cornua. My specimens of Didymodus 

 compressus do not exhibit teeth on the roof of the mouth, as Kner describes. 

 There are no spines with the crania, although separate Pleuracanthus 

 spines are not rare in the same beds. The teeth associated with the skulls, 

 moreover, present a button on the superior side of the root ( Fig. 5). Agassiz 

 figures teeth of this kind as belonging to the Diplodus gibbosus. St. John 

 and Worthen make these teeth typical of Diplodus, and confer the name 

 Thrinacodus on those without the button, a character which I do not think 

 a constant one. The latter name is then probably a synonym of Pleuracan- 

 thus. The button-bearing teeth are figured and described by Kner as occur- 

 ring scattered, and in a somewhat different horizon from that of the Pleu- 

 racanthus specimens. In Germany, as in Texas, the button-bearing teeth 

 are the larger. I suspect that the skull I have described represents a different 

 genus from Pleuracanthus proper. This genus will not differ from Chla- 

 mydoselachus Garm., in the lack of other evidence ; the teeth presenting 

 only specific difference. 



(5.) Of course, a study of the anatomy of Chlamydoselachus, which I 

 hope Mr. Garman will soon give us, may reveal differences between that 

 genus and Didymodus ; but of these we know nothing as yet. 



(G.) The order Xenacanthini was proposed by Geinitz (Dj^as) for Pleu- 

 racanthus, on account of the supposed suctorial character of the ventral 

 fins. This character is supposed by Kner to be sexual. In any case this 

 division, whatever its value, must be subordinated to the order Ichthyo- 

 tomi, as I define it. 



EXPLANATION OF PLATE. 



All the figures two-thirds natural size, except fig. G, which is one- half 

 larger than nature. 



Fig. 1. Skull from above, right frontal bone displaced, and its anterior 

 extremity broken off. Posterior apex broken from right frontal cartilage 

 bone, a, Frontal or supraorbital bone, that of the right side displaced ; b, 

 anterior nostril ; c, postfrontal facet for palato pterygoid ; d, frontal fissure. 



Fig. 2. Posterior part of skull of another individual, from above ; a, occi- 

 pital bone ; b, parietal ; c, a cornua of frontal bone. 



Fig. 3. Anterior view of fig. 2, displaying section of brain case ; a, frontal 

 or parietal cartilage bone ; b, sphenoid ; c, brain cavity ; d, frontoparietal 

 fontanelle ; e, hyomandibular condyle (? pterotic bone). 



Fig. 4. Anterior part of skull from below, of a third individual, display- 

 ing orbits and postorbital processes. 



Fig 5. Tooth of Didymodus compressus Newb., natural size, posterior 

 view. 



