20 REPORT — 1844, 



less amount of tubular perforation, is characteristic of several other families of 

 Lamellibranchiate bivalves, which have the mantle wholly or partially closed ; 

 and it would not, therefore, serve by itself to distinguish a fragment of a shell 

 of this family from those alluded to. But it is quite sufficient to distinguish 

 a shell of this family from any of the neighbouring families, to which, in its 

 general characters, it might possess an affinity. The following is a charac- 

 teristic example of its use : — A shell was described by Prof. Philips, in his 

 ' Geology of Yorkshire,' as an Avicula, which had been previously described 

 by Messrs. Young and Bird as a Pecten. The same species, or one closely 

 allied to it, found near Bristol, was described by Mr. S. Stutchbury as an 

 Avicula ; he not being at the time aware, that it had been met with and de- 

 scribed elsewhere. The mixture of characters is such, as would sanction its 

 being placed in either group, according to the relative value attached to 

 them. Thus, in the form of its hinge it is most allied to Avicula, Avhilst in 

 the flatness of its under valve, and in the disposition of its costse, it rather 

 corresponds with the Pectens. The intimate structure of the shell here 

 sei-ves, I think, to decide the point ; for we find no trace of either the pris- 

 matic cellular substance or the nacre, which are characteristic of Avicula ; 

 but we meet, on the other hand, with the coarsely-corrugated and somewhat 

 tubular structure of the Pectinidce. 



XIII. Margaritacece. 



52. I employ the above designation of this family, because I believe it to be 

 the one most applicable to the genera I include in it, which are the follow- 

 ing : — Perna, Malleus, Crenatula, Vulsella, Avicula and Pinna, with the 

 addition of the fossil genera Gervillia, Inoceramus and (I presume) Catil- 

 lus *. All the genera thus associated together exhibit a remarkable uniformity 

 as to the structure of their shells, — the exterior being composed of prismatic 

 cellular substance, and the interior of true nacre, — both of which structures 

 here present themselves in their most characteristic form. There is no dif- 

 ference whatever, that I have met with, except as to the size of the cells, be- 

 tween the elementary structure of any of these shells. This difference is often 

 very considerable ; thus the average diameter of the hexagonal cells of the 

 large fossil Pinna is about 1-lOOth of an inch, whilst that of the cells of a 

 small (unnamed) species of Vulsella, kindly presented to me for examination 

 by Mr. Cuming, is about 1 -2800th of an inch. One cell of the former would 

 contain, therefore, in its area, about 784 of the latter. In three species of 

 recent Pinna which I have examined, the average diameter of the cells haa 

 been found very nearly the same, namely, 1 -500th of an inch. One of these, 

 however, shoAvs a remarkable difference in the size of the cells at the exterior ■ 

 and interior of each layer, the average of the former being about 1 -380th of] 

 an inch, whilst that of the latter is about 1 -833rd : this difference is due to 

 the fact, that several of the cells of the superficial part of the layer are not 

 prolonged through its thickness, but cease near its middle, as shown by exa- 

 mination of the vertical section, so that there is room for the enlargement of 

 the others. In the genera Perna, Avicula and Malleus, I have found more 

 variation in the size of the cells in the same shell than in the preceding ; a < 

 layer of much smaller dimensions than the average, being generally found 

 where this tissue comes in contact with the nacreous substance (figs. 45-50). 



53. Although the genus Pinna has been placed by nearly all Conchologists 

 in the family Mytilacece, yet I have ventured to associate it with the other 

 genera I have named, on account of its close conformity with them in the 

 structure of its shell, and its entire difference in this respect from the true 

 Mytilacece. And this alteration of its position seems justified by a careful, 



* I have not had an opportunity of examining this genus. I 



