229 



plerumque plus minusve hiante ; appendice ligamenti calcarea pro- 

 longata. 



Long. -56, lat. -92, alt. -34 poll. 



Hab. Mazatlan : legit C. Shipley, Esq. Mus. Cuming, Archer, 

 Darbishire, &c. 



A small species, nearly as glossy as O. nitidum, Gould, from Cali- 

 fornia. In form it resembles L. cuneata, Gray, but is of a much 

 lighter colour and thinner texture, and is not truncated at the anterior 

 margin. The specimens brought by — Thorne, Esq., Paymaster of 

 the Navy, vary greatly in the amount of pedal gape, which in most 

 of Lieut. Shipley's specimens is scarcely perceptible. 



3. Periploma excurva. 



P. t. " P. Leanse" simili, sed minus incequilaterali, margine ventrali 

 maxime excurvato, postice et antice magis hiante, minus aqui- 

 valva, fossa cardinali et clavicula gracilioribus ; magna, tenui, 

 ventraliter arcuata, striis incrementi gracillimis ; epidermide tenu- 

 issima, vix antice rugosa ; cicatricibus muscularibus et pallii mar- 

 gini appropinquantibus , sinu modico, subtriangulato . 

 Long. 2-06, lat. 2-46, alt. 1-05 poll. 

 Hab. Mazatlan, teste Dr. Griiner. Mus. suo. 

 A very fine species, differing from P. Leana, Conr. (not to be 

 confounded with Cochlodesma Leana), and from P. argentaria, 

 Conr., in being much less inequilateral, with the ventral margin well 

 rounded. 



4. Periploma papyracea. 



P. t. suborbiculari, incequilaterali, incequivalva ; parte postica bre- 

 vissima, rostrata, rostro curto, lato; tenuissima, diaphana, albidu ; 

 superficie striis incrementi tenuibus, lineas radiantes hue et illuc 

 vix monstrante, granulis minimis induta, epidermide tenuissima ; 

 umbonibus prominentibus ; marginibus ventralibus et dorsalibus 

 anticis subaqualiter excurvatis, posticis subincurvatis ; fossa car- 

 dinali parva, solida, clavicula solidiuscula ; sinu pallii ore angus- 

 tiore, intus lato, vix angulato. 

 Long. -78, lat. 1-06, alt. -42 poll. 

 Hah. Mazatlan. Mus. Cuming. 



A perfect valve, and a broken pair displaying the hinge in situ, are 

 all that are known of this beautiful species. It differs from the 

 others in its outline, the greater part of which is suborbicular, with 

 a short broad beak. 



5. Thracia squamosa. 



T. t. " T. villosiusculse" simili, sed magis transversa; superficie 

 granulis distinctioribus instructa ; ligamento extus curtiori, intus 

 fulcro majore, minus declivi ; sinu pallii angustiore, magis pro- 

 ducto. 



Long. -72, lat. 1-14, alt. -38 poll. 



Hab. Mazatlan : legit C. Shipley, Esq. Mus. Cuming. 



Remarkably like the British species, from which it differs, — in the 



