44 



Differs from M. Modiolaris in being more ventricose in tlie posterior 

 half, and in having a more deeply impressed byssal sinus, and in having 

 the dorsal and ventral margins more nearly parallel. 



Locality and Formation. — City of Ottawa, Trenton limestone ; also at 

 the Petite Chaudi^re Rapids, two miles from Ottawa, in the Black Eiver 

 limestone. 



Collectors. — J. Richardson, E. Billings. 



MoDiOLOPSis Maia. (N. sp.) 



Pig. 46. 



Description. — Small, transverse, ventricose ; posterior extremity obliquely 

 truncated ; umbones somewhat flattened ; beaks small, closely incurved, 

 almost in contact with each other. Dorsal margin behind the um- 

 bones straight, gradually ascending until about one third of the whole 

 transverse length from the posterior extremity, forming an obtuse angle of 

 about 120° with the posterior margin which descends with a gently curved 

 slope to the posterior angle, the latter obtusely rounded and situated in 

 the ventral thu-d of the shell. Ventral margin nearly straight in the pos- 

 terior two thirds narrowly curved upwards at the posterior angle, more 

 broadly curved up to the anterior angle which is narrowly rounded and 

 situated a httle below the middle of the shell and projecting about i the 

 whole transverse length in front of the umbones. A wide, very shallow 

 byssal smus extends from the beak obliquely backwards to the ventral mar- 

 gin, the middle \ of the length of which it occupies. A strong umbonial 

 gibbossity runs diagonally backwards and downwards, becoming obsolete 

 just before reaching the posterior ventral angle. Between this and the 

 posterior extremity of the hinge hne there is a concave slope. The grea- 

 test • gibbosity is about the mid-length of the shell and nearer the dorsal 

 than the ventral margin. 



Surface not well exhibited in the specimen observed but shewing a few 

 concentric lines of growth. 



Transverse length 6 lines ; umbones to ventral margin 3 hnes ; poste- 

 rior extremity of hinge line to ventral margia 3i lines. 



Locality and Formation. — East of Blue Point, Lake St. John, on the 

 Saguenay. Trenton limestone. 



Collector. — J. Richardson. 



