136 PROCEEDINGS OF THE ACADEMY OF [Feb., 



elevated into an angle that is easily felt, forming a slight keel. The 

 impressed suture is produced by a lowering of the suture line in this 

 part of the last whorl and a wounding of the whorl. This depression 

 of the suture is only to the periphery of the preceding whorl, however, 

 and not below it, as in the typical forms of P. b. pretiosa. In this 

 specimen the umbilicus is only half covered by an expansion of the 

 lip. The other specimen is somewhat larger, measuring 32 mm. in 

 width by 60 mm. in height; the index is .534 — nearly the same as the 

 last. The shell consists of b\ whorls (therefore slightly less than the 

 last), with the suture on the last half of the last whorl only slightly 

 impressed; the periphery in this portion of the shell is flattened, not 

 keeled, and is even slightly impressed. The umbilicus is completely 

 closed by an expansion of the lip. 



Compared whorl by whorl with the specimens that are considered 

 below, either of these runs somewhat larger after the second whorl, 

 up to which point there is little difference in any of these forms. 



Pleurodonte bainbridgei Pfr. Plate VIII, figs. 1, 2. 



At Somerset colony. 



Shell moderately elevated with about 5J whorls, which are more 

 -compressed and rounded in the last 1J whorls than in the Ridge near 

 Lincoln colony, and in fact are the form that has been described as 

 P. b. pretiosa by C. B. Adams. The suture at the end of the last 

 whorl drops below the periphery by 1 or 2 mm. in many cases, and 

 in two that were abnormally developed due to an injury ; the dropping 

 of the suture line amounts to 3 or 4 mm. and extends around the 

 shell for the last half whorl. A number of the shells from this colony 

 were very old, "subfossil," and yet they did not differ appreciably 

 from the fresh living shells, showing that this form has not been 

 interfered with by the clearing, or at least not altered in form by it. 

 These semi-fossil forms were carefully measured, but they give sub- 

 stantially the measurements of the living adult forms of this locality. 

 The average size is: width 53.2 mm., height 33.2 mm., index .62 x 5. 

 The size varies considerably, however, from 48 x 32 mm. to 54 x 34 

 mm. ; the index ranges from .588 to .694. The mean divergence varies 

 from 110° to 115°. 



Pleurodonte bainbridgei Pfr. Plate VIII, figs. 3, 4. 



At the Ridge near Lincoln colony. 



Shell with only moderately depressed spire, consisting of slightly 

 over 5j/ whorls, the suture of the last half of the last whorl but slightly 

 depressed, much less so than in typical forms of P. b. pretiosa C.B.Ad. 



