CONTBIBUTIONS TO PALEONTOLOGY. 31 3 



GENUS LEIORHYNCHUS, Hall. 



At the time of proposing the Genus Leiorhtnchus, I had some ground 

 for believing it referable to the Spiriferidse ; but later examinations have 

 not shown the existence of internal spires, while, so far as I have been 

 able to determine, the hinge-structure is very similar to that of Rhyn- 

 CHONELLA = Stenogisma. The septum in the dorsal valve has a decided 

 triangular pit at its upper or rostral end, while the crura are apparently 

 reduced to short curving processes. Externally the species of Leiorhyn- 

 CHUS may be distinguished from those usually referred to Rhynchonella 

 by the low rounded plications, which are often bifurcating and become 

 obsolescent on the sides and towards the beak of the shell. These very 

 distinctive external characters are, I believe, sufficient to warrant the 

 separation from any other genus, and I have no doubt they will be found 

 connected with internal characters differing from Rhynchonella or any 

 other genus of fossil shells. 



Some of the species are only plicated towards the front, and the mesial 

 fold and sinus are often limited to the lower half of the shell. Even the 

 more distinctly plicated forms are comparatively broader and smoother 

 than any of the Rhynchonellge. When occurring in the same beds with 

 Rhynchonella, the aspect and condition of the shells of this genus are 

 quite different ; and the geographical range of species of the two genera 

 is likewise very different, the Leiorhynchus having a more restricted 

 vertical and horizontal distribution. I have only to regret that all efforts 

 to develop and illustrate the interior structure have thus far proved 

 unsatisfactory. 



Cab. Nat. 40 



