43 



The length from posterior to anterior extremity is about 2 inches ; 

 from greatest elevation of dorsal margin in the posterior half to ventral 

 margin, 1 inch ; from umbones to ventral margm, f of an inch. 



Differs from M. Modiolaris in its greater gibbosity, and from M. Gres- 

 neri in the convex ventral margm, and absence of a byssal sinus. 



Locality and Formation. — City of Ottawa. Trenton limestone. 



Collector. — E. Billmgs. 



MoDiOLOPSis Gesneri. (N. sp.) 



Fig. 45. 

 Fig. 45 — a, View of right valve and h, hinge line of M. Gesneri. 



Description. — Transversely elongate, arcuate, ventricose in the poste- 

 rior half; a wide, shallow byssal sinus extending from the umbones to the 

 ventral margin where it occupies the posterior two-thirds of the anterior 

 half of the shell. Umbones somewhat flattened by the byssal sinus ; beaks 

 closely incurved ; dorsal margin from the umbones to within about one- 

 fourth of the whole length from the posterior extremity gently arched, or 

 nearly straight, and nearly parallel with the ventral margin ; then des- 

 cending with a gentle curved slope to the posterior extremity, which is 

 narrowly rounded ; ventral margin, with the exception of the concave 

 curve caused by the byssal sinus, nearly parallel vai\\ the dorsal. Anterior 

 extremity small, about one-sLxth of the whole transverse length of the 

 sheU. Surface with concentric ridges of growth of variable size, four or 

 five in one line, with stronger ones sometimes two or three lines apart. 



Transverse length, 2| inches ; from the umbones to ventral margin, 10 

 or 12 lines. 



