1911.] NATURAL SCIENCES OF PHILADELPHIA. 143 



may be living in the Balaclava district. They closely approach 2, 

 Somerset gully forms, which are no doubt their descendants. 



2. Somerset Gully Forms (Plate IX, figs. 3, 4). — This is the 

 largest living P. a. goniasmos of this region. The shell consists of 

 about 5^ whorls, the spire being somewhat lower than in the Somerset 

 extinct form. The embryonic shell occupies about If whorls. In 

 form of shell and concavity next to the periphery, as well as in the 

 keeled character of the latter, this shell resembles the extinct form; 

 but in many shells the whorl next the periphery is convex below and 

 even above near the end of the last whorl. This convexity of the 

 whorl next to the periphery above is an accompaniment of diminution 

 in size, and is common to almost all of the smaller forms of this species 

 examined from each of the localities mentioned. As this type passes 

 into the next (3), this character becomes normal in the entire last whorl. 

 The labrum has the outer tooth developed in above 80 per cent, of 

 the specimens, but the inner tooth is only present in some 14 per cent. ; 

 and somewhat less than 5 per cent, are destitute of teeth. Average 

 height 23.5 mm.; width 50 mm.; index .47; mean divergence about 

 130°. 



The form varies with the convexity or concavity of the whorl next 

 to the periphery above; the more typical forms — those present in the 

 largest numbers — are convex at this point on a part of the last whorl. 

 An examination of the young shells shows that up to 4 whorls the 

 form of the shell is that of P. ingens C. B. Ad. or of P. acuta patina 

 (C. B. Ad) ; between 4 and 4J whorls it passes into the form of P. a. 

 acuta and then rather rapidly into the typical P. a. goniasmos form. 



3. Somerset Normal (Plate IX, figs. 5, 6). — These forms resemble 

 the more convex type of the gully form and differ principally in size 

 and in having a somewhat higher index, about .52. They are actually 

 higher than the gully form by one or two millimeters when the 

 width is some three millimeters less. The shell consists of 5| whorls, 

 or a trifle more than in the gully, but the diameter of the whorls is 

 less in every case and the fifth whorl has a diameter of 41 mm., as 

 against 45 mm. in the gully form. The smaller shells, as a rule, are 

 typically convex next to the periphery above, but wherever this 

 character is well-marked the form is flatter and the index slightly 

 less than in those with a concavity of the whorl at this point. Such 

 more convex and less keeled forms generally measure about as given 

 above, the more acute forms measure 24 mm. by 48 mm. to 25 mm. by 

 50 mm., with an index of .50, and thus pass into the gully type. They 

 are then the intermediate stage between the present gully form living 



