VERTEBRATES. 69 



of coalescing of transversely elongate teeth of the younger with the 

 broad long plates of the later stages of growth. The latter feature 

 assumes various phases, from a shallow undulation in the border to 

 a deep slit extending through the thickness of the crown and base, 

 the impinging margins being crenulated as in the inner and outer 

 margin of entire individuals. None of the Kinderhook examples, 

 however, show this latter feature, yet it may be expected to occur 

 in them as well as in the Chester species. A medium-sized tooth 

 of the Chester species affords the following measurements : Breadth 

 across inner margin, 22 mm. ; ditto, outer margins, 13 mm. ; length 

 along anterior border, 13 mm. ; ditto, posterior border, 16 mm. ; 

 thickness of tooth at middle of anterior border, 3 mm. ; ditto, pos- 

 terior border, 4 mm. 



The teeth of the present species differ from European species 

 Psephodus magnus, Agass., as also from the Chester P. crenulatus 

 (N. and W.) in their more symmetrical outline and distinct defini- 

 tion of the angles. The Chester species, perhaps, bears more inti- 

 mate comparison, but it is not known to have attained nearly the 

 size of the European species. The Kinderhook teeth under consid- 

 eration are further distinguished by the comparatively distinct an- 

 gulation of the coronal ridge of the mandibular form. 



PSEPHODUS PLACENTA, (N. and W., sp.) 



PI. II, Fig. 5-8. 

 Helodus placenta, NEWBEEKY and WORTHEN. 1866, 111. Geol. Surv., II, p. 80. PI. V, Pig. 4, 4a. 



Associated with the forms last described under the name Psephodus 

 obliquus, the collections contain a few examples of teeth, which so 

 far as relates to superficial markings and general appearances, 

 might be presumed to have belonged to the fishes that bore those 

 teeth. But in all the collections from Chester localities, where the 

 forms of P. crenulatus abound, there is not a single representative 

 of the form here alluded to. Therefore, in recognition of its dis- 

 tinctive features, it is provisionally referred to a distinct species. 



As at present understood, only two forms of teeth have been 

 recognized as probably belonging to this species. First, median 

 form of the upper jaw (?); Teeth irregularly quadrilateral in out- 

 line, moderately inrolled spirally, anterior and posterior borders 

 differing little in relative length, converging at a slight angle, the 

 narrow enamel fold defined by a slight groove from the basal por- 

 tion, which shows coarse verrucose markings ; inner margin broadly 



