142 PROCEEDINGS OF THE ACADEMY OF [Feb., 



from 44 mm. to 48 mm., inclusive, representing the normal forms. 

 At a diameter of 49 mm. we find 26 forms; at 50 mm., 19 forms; at 

 51 mm., only 7 forms. There are a few scattering normal forms in 

 these 52 gully forms and a few of the gully forms above 51 mm., up to 

 54 mm., making the number distinguished as gully forms up to 55. 

 The 12 extinct specimens collected range from 54 to 60 mm. in width. 



The curve of the heights does not serve so well to sort out the four 

 types, for exclusive of the extinct type there is not so much range in 

 height among the specimens except those of the gully type, which vary 

 in height from 28 mm. to 23 mm., though mostly falling between 

 27 mm. and 25 mm. The curve shows a rapid rise from 19^ mm., of 

 which only one was recorded through 20 mm., of which only 2 examples 

 were measured to 23 mm. as a maximum where the number recorded 

 was 54. Between the heights of 22 mm. and 25 mm. there are included 

 108 specimens. As has been stated, a small number of characteristic 

 examples of each of the three living types were selected for measure- 

 ment by whorls and all of the perfect specimens of the extinct type 

 collected were used; these measurements were employed in the 

 descriptions which follow. 



1. Somerset Extinct (Plate VIII, figs. 1, 2). — Shell rather more 

 elevated than in the Somerset gully forms, consisting of 5| whorls, 

 of which the first two belong to the embryonic shell; with an acute 

 periphery which continues to the end of the last whorl; slightly 

 concave next the periphery below up to the beginning or sometimes 

 the middle of the last whorl, somewhat concave next the peripheiy 

 above to the end of the last whorl; labrum with one outer tooth on 

 the lower side, and sometimes with a second inner tooth; umbilicus 

 normally closed by expansion of the lip. Average height 31 mm.; 

 width 58 mm.; index .535; mean divergence about 115°. 



About 60 per cent, of the shells had one tooth, the remaining 40 

 per cent, had two more or less well-developed teeth. This is a difference 

 from the other Somerset forms now living, which showed 170 out of 

 209 with one tooth and ten with no teeth, against 29 with two teeth, 

 of which the second inner tooth was often only a trace. 



These Somerset extinct were found in several favorable localities, 

 but were only collected from one cockpit on the little hill of honey- 

 combed limestone that has been already mentioned. A few specimens 

 were taken (about a dozen), although hundreds were seen here 

 and elsewhere on the estate. It is assumed that they are extinct, 

 as no fresh shells of this large high type were observed ; but a 

 protected gully may be found where they are still alive, and they 



