44 



Differs from M. Modiolaris in being more ventricose in the 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 ChaudiSre Rapids, two miles from Ottawa, in the Black Biver 

 limestone. 



Collectors. J. Bichardson, E. Billings. 



MODIOLOPSIS MAIA. (N. sp.) 



Fig. 46. 



Description. Small, transverse, ventricose ; posterior extremity obliquely 

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

 almost hi 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 hi 

 the ventral third 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 little below the middle of the shell and projecting about the 

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

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

 gin, the middle i 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 line 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 lines ; poste- 

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



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

 Saguenay. Trenton limestone. 



Collector. J. Richardson. 



