VEETEBEATES. 



Geological position and localities : Upper Burlington limestone, ' 

 bed;" Buffington creek, Louisa county, Pleasant Grove, Augusta 

 and Burlington, Iowa ; Cedar creek, Henderson county, and Quincy, 

 Illinois. 



PSAMMODUS TUMIDUS, St. J. and W. 



PI. XIV, Fig. 1-4. 



The collections contain several examples representing two forms 

 corresponding with the supposed mandibular and maxillary teeth of 

 the foregoing species, Psammodiis Springeri, of the same deposits, and 

 which, presenting apparently persistent characters by which they 

 may be distinguished, we are led to provisionally indicate under a 

 distinct specific appellation. Presenting the same general outline, 

 they are distinguished from P. Springeri principally by the greater 

 extent of the truncated angle of the inner articular border and the 

 distinct longitudinal rugosities that make a highly wrought orna- 

 mental belt usually extending a short distance forward from the 

 posterior margin, both of which characters are common to the two 

 forms. The short, broad mandibular teeth do not differ otherwise 

 to any marked extent from the same form of the above-mentioned 

 species, unless the crown shows a somewhat greater transverse con- 

 cavity and more strongly arched contour from within outward; 

 however, the prominence along the inner lateral border is somewhat 

 more distinctly denned by longitudinal furrows than is observed in 

 the corresponding teeth of P. Springeri. But in respect to the max- 

 illary form, the most striking contrast is noticeable in the latter 

 particular. The inner coronal prominence becomes strongly devel- 

 oped, presenting a distinctly denned lobe, laterally rounded with 

 steep declivity falling to the depressed median area, which in some 

 instances is quite strongly plicated longitudinally in addition to the 

 rugose belt ornamenting the crown surface immediately along the 

 posterior margin; in worn examples the rugose sculpturing is obso- 

 lete. In the majority of instances the limit of the inner coronal 

 ridge is defined by a narrow impressed line. The lesser coronal 

 ridge along the exterior lateral border bears intimate resemblance to 

 P. Springeri, and the same observation may be applied to the super- 

 ficial structure of the teeth. 



At the outset we were inclined to regard the teeth above referred 

 to as merely variations from the normal condition of Psammodus 

 Springeri. Indeed a more extensive suite of material may possibly 



