Sculpture of 

 the surface. 



216 



Dalman's description on the other hand, applies best to a variety atrypo- 

 ides. He, like Wahlenberg, appears to describe only the ventral valve. 



Leopold von Buch, while recognizing the distinctness of the two valves, 

 ascribes a sulcus to the ventral valve which apparently has not been 

 observed by any other writer. With this exception, his description 

 applies to the type of the species ; that is, the evenly lenticular form 

 with a short hinge line. The extreme thinness of the shell fully bears 

 out Dalman's remarks on this point, and is associated with inconspicuous 

 hinge teeth and dental plates. The internal markings of the shell pro- 

 duced by the attachment of the muscles and the ovarian spaces are only 

 faintly indicated and often quite undiscernable. 



Sculpture. There is a wide variation in this respect in examples of 

 different ages, and in the different varieties. As a rule the young shells, 

 especially the ventral valves, are smooth, with only slightly marked 

 diverging ridges. In the Acadian examples of this species the sculpturing 

 is always more distinct on the dorsal than on the ventral valve ; this I 

 find is also the case in the Swedish examples, for which I am indebted to 

 Dr Lindstrom. As the shells grew larger the ribs became more distinct ; 

 and also more numerous by the intercalation of new ribs towards the 

 margin. L. von Buch gives 18 to 20 as the full number of ribs in the 

 adult. This is the usual number in the St. John examples, though a few 

 show as many as 30 ribs. 



Size. The largest example of this species observed at St. John is 8 

 mm. long and 1 1 broad ; but Brogger mentions that an example from 

 Vestfossen was 15-5 x 12-5. Dr Lindstrom has not seen any so large. A 

 medium sized shell of 5 x 6 mm. is by far the most common in rocks of 

 Eastern Canada. 



Horizon and Locality. This species is found in limestone lentiles 

 enclosed in the black shales of Division C. 3 a at Germain street, St. John 

 (also in similar shales of C. 3 6, on King Street, and elsewhere) occurring 

 together as in Sweden, in great numbers and of all sizes. 



I have not observed the Cape Breton examples to vary from the above 

 descriptions. In that island the fossil has been found at the localities 

 named below. 



Horizon and locality. At McNeil brook, east of Mira river in Band 

 C. 3 b ; at Gillis Brook, on East Bay and at McLeod brook, Boisdale, 

 probably in the same assise, though possibly in Assise C. 3 c. 



VARIETIES. PI. XVI I., figs. 2 a-c, 3 a and b and 4 a and b. 



Varieties. Among the forma found at St. John are three which also may be looked 



for in Cape Breton, viz. : lyncioides, with strong radiating ribs, especially 



