66 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



After summing up the characteristics of this group of shells, Mr. Da- 

 vidson remarks : " Such are the general dispositions presented by the 

 " shells composing this subgenus, and which denote its intermediate position 

 " between Orthis and Strophomena." 



From the observations already made, taking these ^^ intermediate^^ cha- 

 racters for our guide, and adding likewise the Genus Strophodonta to 

 Orthis and Strophomena in the comparison, it seems to me that the 

 Genus Streptorhynchus may be much increased in number of species, 

 and a know^ledge acquired of its more extended geological range, while its 

 congener Strophomena will be relieved of its present burden of hetero- 

 geneous material. The result will tend to more accurate discrimination of 

 those groups which we term genera and subgenera ; while at the same 

 time we shall not fail to notice their convergence in certain directions, or 

 among certain forms, and their divergence in others. 



In the lower rocks we have a comparatively great variety of exterior 

 form, and of interior development, among those fossils which have been 

 referred to Strophomena ; but it is by no means easy to separate these 

 into generic groups. It is only in the higher rocks that we have, as it were 

 coming out of these lower ones, the better defined generic forms of Stro- 

 phomena, Strophodonta and Streptorhynchus. Taking for our guide 

 the characteristics of Streptorhynchus as shown in Devonian and Car- 

 boniferous forms, we are able to trace the same features, though less strongly 

 developed, in the older species. 



Applying these characters, we recognize certain of the latter as appa- 

 rently more nearly related to Streptorhynchus than to Strophomena ; 

 and we have three types which may be included under that designation. 

 First, those resupinate forms of the Lower Silurian strata, which have 

 bifid cardinal processes directed outwards, and a strongly limited ventral 

 muscular impression. Second, the Upper Silurian doubly convex and plano- 

 convex forms, similar to Orthis in external characters, and having a broad 

 flabelliform ventral muscular impression : these extend likewise into Devo- 

 nian and Carboniferous rocks. Third, the remarkable group with the ventral 

 valve much elevated, the beak bent or distorted, and the muscular impres- 

 sion strongly limited by the extension of the dental lamellae, and sometimes 

 divided by a strong septum. 



These views, resulting from the detailed comparison of parts among these 

 fossils, may perhaps be more philosophically expressed by saying that in 

 the older forms we have the elements, or the partial and rudimentary ex- 

 pressions, which, in later periods, becoming farther developed, afi'ord the 

 means of dt signating with more clearness and precision the limits of generic 

 and subgeiieric groups. 



