140 



PORAMBONITES OtTAWAENSIS. (N. Sp.) 



Pig. 117. 



Fig. 117. — Porambonites Ottawaensis. a, 6, c, d, Different views of a specimen; 

 e, interior of ventral valve ; /, interior of dorsal valve ; g, side 

 view of a specimen, which, being empty, shows the form of the 

 oral arms. 



Description. — Ovate or sub-triangular ; valves of nearly an equal 

 length ; apical angle about 90° ; sides straight for a little more than 

 one-half the length from the beaks, in the front half rounded ; front 

 margin gently convex, or with a portion in the middle either straight or 

 concave, according as it is affected by the mesial fold and sinus. Ventral 

 valve convex, most elevated at one-fourth the length from the beak ; a 

 concave mesial sinus commences at the front margin and extends in some 

 specimens nearly to the umbo, in others dying out at two-thirds the length 

 of the shell; area small, scarcely one-third the width of the shell, strongly 

 concave, overhanging the hinge-line ; beak small, incurved down to one-third 

 the gTcatest elevation of the valve ; foramen triangular, its width about 

 equal to its height. The form of the dorsal valve is similar to that of the 

 ventral, but it is a little larger, and has a mesial fold which in general is 

 only slightly elevated ; area not qu.ite so high as that of the ventral valve, 

 and lying nearly in the plane of the margin, but a little curved over the 

 plane ; foramen triangular, open to the beak. On each side of the umbo 

 both valves are abruptly deflected, so that they descend to the margin at 

 a I'ight angle. Surface ou the umbo and flat space on each side, smooth; 

 three obscurely angular ribs on the mesial fold and two on the sinus ; from 

 three to five ribs on each side ; none of the ribs reach the beaks. A few 

 squamose zigzag concentric strife are visible. 



In the interior of the ventral valve, the dental plates form two thin 

 vertical septa, extending about one-fourth or one-third the length of the 

 shell. The teeth are well developed. The umbo between the dental 

 plates is hollow quite to the beak, as in Rhynckonella. There is no trace 

 of a deltidium. 



In the interior of the dorsal valve, the bases of the oral arms consist of 

 two short plates, concave on their upper, and convex on their lower sides. 



