MAZATLAN UNIVALVES 183 



times throughout. The apex in the adult is always flattened. 

 The smallest specimen found, '03 in length, has much -the 

 aspect of a Navicella. The apex is terminal, spiral, of a very 

 few whirls, involute, like a tumid Planorbis, turned a little 

 from the middle, away from the canal, and apparently sinistral. 

 The head mark is at the opposite end of the shell. A partition 

 is soon placed across the spiral part, which then drops off at a 

 variable period, leaving a horse- shoe cicatrix. In the speci- 

 mens examined, the angle of adherence varied somewhat, as 

 did the degree of revolution of the spire. The family is thus 

 separated by another character from Patellidae, in which the 

 apex is never spiral, and from the true Calyptreeidse in which it 

 is persistent. The first portion of the normal shell which is 

 formed is smooth. Presently fine striae are developed ; ' after- 

 wards, often quite suddenly, large ribs. There is nothing in 

 the juvenile shell to shew whether it will develop into fhe 

 flat or the conical form. The channel is not apparent in the 

 very young shell; but soon becomes developed to an abnor- 

 mally , large size. The largest shell of the flattened form 

 (including palmations) measures .long. "96, lat. '89, alt. *18. 

 A subconical sp. '76, '65, "24. 



The flattened form, which is rare, may be distinguished as 

 S. L. var. PALMATA : t. planatd, costis magnis, rotundatis, ad 

 marginem projicientibus, intus cavatis, quasi palmatis. It is 

 probably the S. denticulata of Mke. (non Quoy & Gaim.) Zeit. 

 /. Mai. 1851, p. 38, no. 136. 



Hal>. Mazatlan, Philippi ; do. Melchers.; do. abundant ; 

 L'pool Col. 



Tablet 833 contains the youngest .specimen, Navicella-shaped, 

 and an older one with the scar from the detached spiral part. 

 834, 8 sp. jun. different ages, with spiral part visible. 835, 

 9 sp. do. spiral part detached. 



Tablet 836 contains 3 sp. var. palmata, very few (11 + ) ribs. 

 837, 4 do. large and regular. 838, 5 do. irregular. 839, 4 do. 

 ribs very numerous, (22,) smaller. 840, 4 do. interstices of 

 palmations slightly crenate. 841, 5 do. interstices crenated. 

 842, 4 do. greenish tinge. 843, 5 do. light brown. 844, 4 do. 

 very dark. 845, 3 do. nearly black. 



Intermediate form ; slightly palmated margin. Tablet 846 

 contains 3 sp. approaching var. palmata. 847, 3 do. dark 

 brown. 848, 3 do. white margin. 849, 3 do.. light brown, 

 850, 2 do. chocolate colour. 



