266 THE DEVONIAN FOSSILS OF CANADA WEST. 
species known, the ventral valve has an area more or less developed. 
The valves articulate by teeth and sockets. . 
The genus Rensseleria (Hall) is closely allied to Stricklandia, the 
shells being of nearly the same shape and size. Prof. Hall has shewn 
that in the dorsal valve the calcified processes, in his genus, after 
being prolonged about two-thirds the length of the valve, are united 
so as to form a loop, (as in Centronella) with a backward projecting 
spine. I think it probable that when better specimens are procured, 
it will be found that Stricklandia has a similar loop. In Rensseleria 
there is no triangular chamber in the ventral valve. 
This group of shells, (Stricklandia), although closely related to 
Pentamerus, differs from that genus in the following particulars :-— 
Ist. In Pentamerus the form is globular and the ventral valve much 
the largest. In Stricklandia the valves are nearly equal in size, and 
the form oval or heart-shaped, never globose. 2nd. In Pentamerus 
the dorsal valve has two and sometimes three well developed longitu- 
dinal 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. galeatus, P. Sieberi, P. acutolobatus, P. 
caduceus, &c., the difference in the external form of the two groups 
is so remarkable, that we would 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. 
