420 MAZATLAN UNIVALVES 



plied decUvi intus conspicuo extus ad labrum sinuatum eircum- 

 eunte ; operculo tenuissimo, vix spirali, striulis rugosis radian- 

 tihus oniato. 



As no fewer than 500 specimens were found of this typical 

 species, (a rare number for a PjTamidellicl,) most of them in 

 very fresli condition, it is possible to describe it witb tolerable 

 accuracy. It is distinguished from its neighbours by the basal 

 strise which are numerous, close and fine, beautifully adorned 

 between with dots more or less elongated. The outer lip is 

 produced and broadly indented at the base, narrow above and 

 pinched at the suture, where it joins the labium which is 

 often rather separate from the body whirl. The plait winds 

 round the axis and develops into the boundary of the basal 

 sinus. The sutural groove is not carried over the base, but is 

 formed simply by the transverse ribs ending just before they 

 reach the next whirl. These are not parallel to each other, 

 but to the diverging outlines of the spire. They are very 

 conspicuous on the young shell, which is broad and rather 

 angidated at the periphery. As the adult shell narrows itself 

 in, the base is produced and rounded, the ribs become some- 

 what indented by the spiral lines, and near the mouth fade 

 away altogether. In the adolescent state the labrum is often 

 beautifully serrated hj the spiral stria; ; in the adidt the lip is 

 still transparent in the living shell. Its great thinness causes 

 it to be frequently broken and repaired, the fresh part being 

 not unfrequently almost devoid of sculpture. The spire is 

 sometimes encrusted with coralline during the life of the animal. 

 The operculum, (traces of which were observed in only about 

 a dozen specimens,) is large, extremely thin, transparent, 

 sometimes shewing a shghtly spiral element, and covered over 

 almost the whole surface with most minute wrinkles, radiating 

 incurves from the centre like the engine-turning of a watch. 

 Not only the size, but the number of whirls varies in different 

 specimens known to be adult by the shape and evanescence of 

 the sculpture. Very few are so large as those from Panama, 

 and they generally make one turn less. The smallest specimen 

 measures '03 by '021; the smallest adult, 'OoS by '025; an 

 unusually large one, long. '12, long. spir. '085, lat. (in med.) 

 •048, div. 23°. 

 Hah. — Panama ; 00 sp. under stones near low water mark ; 



C. B. ^rfam*.— Mazatlan ; abundant and fresh among algse 



on Uvanilla, somewhat rare on Chama, Spondylus, &c. ; 



L'pool Sf Havre Coll. 



