GUELPH FAUNA IN THE STATE OF NEW YORK IOI 



This completeness of the material at hand gives basis for the amplifi- 

 cation of the characters of the species which have heretofore been drawn 

 solely from the immature conditions of the shell. 



Shell dextral, forming a low torticone of about two and one half volu- 

 tions, not quite one half of the last volution being occupied by the living 

 chamber. The volutions expand and coil downward very gradually, so 

 that the apex of the shell and the upper side of the outer chamber lie in 

 almost the same horizontal plane. The section is described by McChes- 

 ney as being subelliptic with the dorsoventral diameter greater than the 

 lateral, while Hall states that the volutions in these immature examples 

 are " essentially circular." The Rochester specimens are subcircular in 

 the section of the early part of the last whorl but become laterally flat- 

 tened in later growth, so that in the final stages the cross section is ovate. 

 The ventral side is subacutely and the dorsal obtusely rounded, but the 

 latter is not flattened nor does it bear any trace of an impressed zone. In 

 gerontic specimens the sides become flattened to parallel planes, a fact 

 mentioned by Hall as characteristic of the genus. The siphuncle, which is 

 described and figured by Hall as being central, is centren in the nepionic 

 whorl, becomes however, in the mature stage, as shown by several of the 

 Oak Orchard creek specimens, ventrocentren. The ventral position of 

 the siphuncle is given by Hyatt as a family character of his Plectocera- 

 tidae, to which this genus appertains. The siphuncle is small, apparently 

 tubular. The camerae are of slight depth and the septa closely arranged 

 (the final ones 3 mm apart), the suture has a prominent saddle on the 

 venter and a broader and lower one on the dorsum. The lateral lobes are 

 shallow and broad, deepest near the dorsolateral curve. 



The surface sculpture consists, in the earlier stages, of angular, 

 oblique ridges, with sliglicly concave to flat interspaces. These bend 

 strongly forward on the dorsal and backward on the ventral side, thus 

 intersecting the suture lines at a considerable angle. They become faint as 

 growth advances and on the last half or two thirds of the final whorl are 

 obsolescent, in old shells disappearing before septation ceases. Concentric 



