158 



BRITISH SILURIAN BRACHIOPODA. 



and the ventral valve much the largest. In Stricklmidia the valves are nearly equal in 

 size, and the form oval or heart-shaped, never globose. 2nd. In Fentamerus the dorsal 

 valve has two, and sometimes three, well-developed longitudinal septa, which in most of 

 the species sustain a small triangular chamber, as in the ventral valve. In Stricklandia 

 these septa are not developed, and the triangular chamber is entirely absent. It might be 

 thought that the difference between the short or rudimentary socket-plates of Stricklandia 

 and the elongated mesial septa of Pentamerus should not be regarded as of sufficient 

 importance to constitute a generic distinction, because it is only a difference in the extent 

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

 rudimentary state 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. 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 of species, then 

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

 them with the ordinary forms of Pentamerus — such, for instance, as P. Knightii, P. ga- 

 leatus, P. Sieberi, P. acutolobatus, P. caducus, &c. — the difference in the external form of 

 the two groups is so remarkable that we should almost be 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." 



In the first volume of * Palaeozoic Fossils,' pubUshed by the Geological Survey of 

 Canada, p. 84 (1865), Mr. Billings further remarks — "This genus was proposed by me 

 to include such shells as those known in England under the names of Pentamerus lens, 

 P. liratus, and P. lavis. They differ from Pentamerus in having the valves usually sub- 

 equal, and no longitudinal septa or triangular chamber in the interior of the dorsal valve. 

 Both valves have an area, but in the dorsal it is usually linear, or only slightly exceeding 

 the thickness of the substance of the shell in height. The ventral valve has usually a 

 concave mesial sinus more or less developed, and the dorsal valve a mesial fold corre- 

 sponding thereto. The hinge-line in some of the species, such as in S. Icevis and S. micro- 

 camerus, have the hinge-line straight and much extended."^ 



^ Mr. Billings is in error in including Pentamerus Icevis in Strichlandinia, that shell being the young 

 of Pentamerus oblongus. 



