RHYNCHONELLIDiE OF THE TULLY LIMESTONE. 347 



which are often obscurely defined. In old shells there are forty or 

 more rather broad, rounded, moderately elevated plications, of which 

 from six to eight are depressed in the sinus, and a corresponding num- 

 ber elevated on the mesial fold. These, as well as some of the lateral 

 plications, are usually bifurcated above the middle, but occasionally 

 below. Those near the margin, and particularly on the fold and sinus, 

 are flattened and longitudinally grooved. The plications are crossed 

 by fine concentric striae which are not visibly imbricating. 

 The casts show a small rostral cavity, bordered by short and rather 

 thin dental lamellae, with an elongate-ovate muscular impression, and a 

 moderately or scarcely elevated septum in the dorsal valve. The surface 

 shows a beautiful ramified system of vascular markings, the main trunks 

 of which curve around .the papillose ovarian spaces, and thence bifurca- 

 ting are distributed according to the divisions of the plications, giving 

 a branch to each one, and these again usually subdivide towards the 

 extremities. The central and subordinate lateral groovings of the exte- 

 rior of the plications correspond to the vascular markings and their 

 subdivisions. ' 



The smaller specimens measure sometimes less than half an inch in 

 length and breadth ; while the full-grown shells measure from seven- 

 eighths to nearly an inch in length, with a width sometimes exceeding 

 an inch, and a depth frequently of three-fourths of an inch. 



This beautiful fossil occurs in considerable numbers in several localities of the 

 •Tally limestone, and in full-grown individuals is readily distinguished ; but the 

 immature phases are not so easily recognized. 



This, species was identified by Mr. Vanuxem with the Terebratula cuboides of 

 SowERBT ; and the same name with doubt was used in the Keport of the Fourth 

 District. M. A. D'Orbigny has separated the species originally described by 

 SowKRBY from that described under the same name by Philips (loo. cit.), propo- 

 sing the name Atrypa svbciiboides for the latter, citing at the same time the 

 American species as identical. 



The British species has a different aspect and has more plications on the mesial 

 fold and sinus, which is proportionally more elevated in front ; while in the exfo- 

 liated shell all the plications are more angular. It becomes necessary, therefore, 

 to propose a distinctive name for the American species. Although having a geue- 



