288 MAZATLAX UNIVALVES 



Comp. Calyptra?a gemmacea, Val. Voy. Ven. pi. 15, f. 2. [If 

 the figure be accurate, this is a distinct, tubercled species : 

 but it may be a small, conical, worn C. imbricata ; the quasi- 

 tubercles being worn ribs.] 



The gifting of the intercostal spaces which seems to form the 

 groimd of separation between C. imbricata of Soto, and C. 

 dentata of ][fke. appears so very variable a character, that I 

 have not ventured to regard them as distinct. The Mazatlan 

 specimens (when in sufficiently fine condition to speak with 

 certainty) are never wholly without pits : yet they are com- 

 pletely irregular in their formation, often not being seen over a 

 great portion of the surfece ; not unfrequently appearing on one 

 side only ; and very rarely traceable on the young shell. The 

 species is normally rather solid, conical, with a very variable 

 nimiber (about 20) of stout rounded ribs, more or less spread- 

 ing out at the margin. Fresh ribs are frequently formed in 

 the intercostal spaces. When a fresh margin is formed below 

 the palmations of the former one, a series of pits is produced. 

 As the palmations are most developed in the adolescent state, 

 so are the pits. They are rare in the older growth. Both ribs 

 and intercostal spaces are finely, but irregularly indented with 

 radiating coi-rugations. Fine radiating brown lines are often 

 traceable outside, dotting the interior margin. The surface 

 is generally rough and often covered with accretions; the 

 vertex is rarely jjreserved. 



In Mr, Darbishire's collection, is a specimen with the animal 

 matter dried within, and the young fry covering the principal 

 part of the inner surface of the shell. These are shaped like 

 Vanikoi'o, about "035 across; with the apex flat, not sunken ; 

 "wdth one tumid whirl exposed, and a very large umbilical area. 

 The surface is concentrically and very finely fm-rowed. Most of 

 the specimens have made a faint lip over the base, with an edge 

 which is to commence the margin of the PateUiform shell ; but 

 the cup is not begun. In this stage, the shell cannot be dis- 

 tinguished geuerically from Crepidula. The nuclear whirls are 

 turned laterally with the plane of after growth (as in Capulus), 

 perpendicularly to the base of the shell. The smallest specimen 

 found in the adolescent stage is ' 17 across ; at which period 

 it greatly resembles the young of Crepidula dorsata, the ribs 

 not being developed. The cup is then semicircular, produced 

 in front, adhering at the two separate margins. As it inweases 

 in size, the anterior margin bends round till it joins the 

 posterior one, forming an entire cup, adherent almost to the 



