228 MAZATLAN UNIVALVES 



wing, so that the expansion of each penultimate whirl lies 

 closely over the next. A depressed portion, bounded by two 

 spiral lines, appears between the body and the wing, forming 

 a -false suture on the spire. Base with very fine strise of 

 growth ; with a nodulous spiral raised keel about the middle, 

 and a nodulous line within, bounding the umbilical region. 

 This is of a lustrous orange red, shading into black over a dull, 

 sometimes slightly bifid spiral subcentral keel. The umbilical 

 mark is but moderately excavated, with a slight central cal- 

 losity. Labrum extending one-fourth of a whirl. Parietal 

 labium covering the inter-carinal region for another fourth. 

 The rest of the shell (base and spire) is covered with an 

 oblivaceous epidermis, lying in strongly striated lines of growth. 

 Colour beneath the epidermis dark green, more or less tinged 

 with red, especially in the young shell. The medial portion, 

 wing and tubercles, are often very slightly shewn in the adult. 

 The shell when in good condition is extremelv beautiful ; but 

 it is almost always covered with animal and vegetable incrus- 

 tations. It is subject also to the attacks of Lithophagi and 

 Gastrochsenae, which generally have the instinct to burrow 

 along the thick winged portion, or down the axis ; but often 

 venture to invade the liver-chamber in the upper whirls, when 

 the Trochid withdraws itself, forming partitions as in Turritella, 

 &c. Operculum smooth, outside with two strong ridges, begin- 

 ning from nearly the same point, and meeting at the other 

 end ; of which the outer is far larger ; with a deeply-scooped 

 hollow within ; and a faint ridge at the outer margin. Inside 

 obtusely raised at the back of the large ridge ; with blackish 

 brown horny covering. The largest sp. measures long, (from 

 apex to wing at base) 2*9, lat. '26, div. 65. A spreading sp. 

 long. 1'9, lat. 2 '15, div. SOP. 

 Hob. Mazatlan ; extremely abundant ; ISpool Col. S. W. 



Mexico, P. P. C. 



Tablet 1088 contains 5 sp. different ages, spreading var. 

 1089, 5 dp. usual form. 1090, 3 do. very conical. 1091, 4 sp. 

 after going through the acid process, with the beautiful basal 

 epidermis removed, but displaying the exterior markings. 

 1092, 4 sp. with base beautifully preserved. 1093, 2 sp. mended 

 after fracture. 1094, 1 sp. with the mouth curiously distorted 

 by an Ostrea conchaphila. Each party has raised its lip to 

 escape the other. 1095, 1 sp. with Lithophagus aristatus in 

 situ. Having bored through an empty barnacle as far as the 

 interior, it has increased outwards, making an external tube. 



