New Genera and Species of Brachiopoda. 133 



merits, 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 sufficient importance to constitute a generic distinction, because 

 it is only a difference in the extent to which identical parts are 

 developed, the dental plates of the former genus being a rudi- 

 mentary 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 modifica- 

 tions 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 developement 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 of 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. galeatus, P. Sieberi, P. acutolobates, 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 dis- 

 similarity in the general form we add the difference in the inter- 

 nal 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. Icevis. All these, and the three Canadian species, abound in 

 rocks of the a^e of the Middle Silurian, such as the Landoverv 

 rocks of Sir R. Murchison, and the Clinton and Niagara groups 

 of the New York geologists. No species have as yet been found 

 either above or below the Middle Silurian. On the other hand, 

 the genus Pentamerus occurs more or less frequently in all for- 

 mations from the Black River limestone* to the Devonian inclu- 

 sive. 



* I have ascertained that Atrypa hemiplicata (Hall) is a true Penta- 

 merus. 



