Mr. J. Morris on the genus Siphonotreta. 317 



chief character consists in a short, perfectly straight, perforated 

 beak, never bent towards the ventral valve. The walls of this 

 beak are very thick, and hence it does not appear, as for instance 

 in the Terebratula, hollow within, but solid and perforated by a 

 narrow sipho, which serves for the reception of a cylindrical 

 muscle of attachment. 



" The beak presents two chief diversities of form : it is either 

 drawn away, in very different degrees, from the hinge-margin 

 towards the centre of the dorsal valve; that is, is placed at a 

 greater or less elevation above the hinge-margin, — or it lies ex- 

 actly in the same plane with the hinge-side of the dorsal valve. 

 In the first case the dorsal valve has properly the form of a cone 

 more or less inclined towards the hinge- side, and the sipho ap- 

 pears either as a perfect tube [Siphonotreta, Acrotreta), or as a 

 tube opened up externally for a portion of its length from the 

 apex of the cone (Schizotreta). In the second case the dorsal 

 valve represents only the half of a cone, in which the shorter 

 hinge or posterior part has been cut away from the apex to the 

 basis, exactly in such a manner that the external opening of the 

 beak is changed into a groove less than a semicircle in depth, 

 and the sipho into a semicylindrical groove open along the whole 

 length of the hinge-surface (Aulonotreta) . See PL VII. 



" In no portion of the shell of this group can we observe the 

 slightest indication of a predominance of development ; the cen- 

 tral part is not distinguished from the marginal portions ; hence 

 neither valve shows either a carina or a sinus ; the hinge-sides 

 form together an arch, and pass imperceptibly into the lateral 

 margins; there are no wing-shaped expansions of the hinge- 

 margins, and finally, neither the cardinal nor anterior margins 

 exhibit either folds, serratures, or excision. 



" The anatomical structure of the shell of the Siphonotretese 

 is this. The whole inner surface is covered by a continuous layer 

 which is so thin that it welds itself closely to, and takes the form 

 of, all the larger prominences and folds of growth. This layer, 

 from its position and colour, I shall call the nacreous-layer (Perl- 

 mutterschicht) . The external surface of the shell is also covered 

 by a continuous, but considerably thicker, corneous epidermal- 

 layer, which is so much developed, and from its horny texture 

 has so great durability, that sometimes, even when all the other 

 layers are dissolved and vanished, it is still perfectly preserved — 

 a peculiarity which, in the whole family of Brachiopods, is found 

 only in this group and in the Lingulce. This epidermal-layer also 

 covers the inner wall of the sipho in all its diversity of forms. 

 Lastly, the part between these two layers, and always the thickest, 

 is the proper calcareous shell" 



Any remarks upon the above characters must be considered 



