80 



-ventral valye supporting a small triangular chamber beneath the beak as in Pentamerm ; 

 in the dorsal valve no longitudinal septa, spires, or loop, the whole of the internal solid 

 organs consisting of two very short or rudimentary socket plates, which in some species 

 bear prolonged calcified processes for the support of the cirrated arms. In some of the 

 species the ventral valve has an area more or less developed. 



" This group of shells, although closely related to Pentamerus, differs from that genus in 

 the following particulars : — 1st. In Pentamerus the form is globular and the ventral valve 

 is much the largest. In Stricklandia the valves are nearly equal and never globose. 2nd. 

 In Pentamerus the dorsal valve has two or three longitudinal septa, which in some species 

 sustain a small triangular chamber. In Stricklandia these characters are entirely absent. 

 It might be thought that the difference between the short or rudimentary dental plates of 

 Stricklandia and the elongated mesial septa of the dorsal valve of Pentamerus is not of suf- 

 ficient importance to constitute a generic distinction, because it is only a diflference in the 

 extent to which identical- parts are developed, the dental plates of the former genus being a 

 rudimentary state of the septa of the latter. When, however, we examine any group of 

 closely allied genera we find that all the grounds for separation consist in the various 

 modifications of the same set of organs. Were it not so then there would be no such thing 

 as^homologous parts. The difference in the degree of the development of an organ is not 

 always a good character, but when it is carried to such an extent that the whole form of 

 the animal is affected in a particular manner, manifested in a number oT species, then it 

 becomes of generic value. If we take the several species of Stricklandia and compare 

 them with an equal number of species of Pentamerus, such for instance as P. Knightii 

 P. galeatuSj P. Sieberi, P. acutolobatus, P. caduceus, &c., the difference in the external form 

 of the two groups is so remarkable that we would be almost warranted in separating them 

 into two genera upon this ground alone ; but when to the dissimilarity in the general form 

 we add the difference in the internal structure then there can be little doubt as to the 

 correctness of the separation." 



" This genus includes three English species which have been long known under the 

 names of Pentamerus lens, P. liratus, and P. Ixvis. All these, and the three Canadian 

 species, abound in rocks of the age of the Middle Silurian, such as the Llandovery rocks of 

 Sir R. Murchison, and the Clinton and Niagara groups of the New York geologists." 



The following figures exhibit the diff'erence in form between Stricklandinia and Pentame- 

 rus : — 



The main differences between Pentamerus and StricJdandinia thus 

 occur in the internal structure of the dorsal valves, discovered by Mr. 

 Salter, and in the general form first pointed out by myself. 



