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 Calli- 
fornia. In form it resembles L. cwneata, 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. PeERIPLOMA EXCURVA. 
P. t.“ P. Lean” simili, sed minus inequilaterali, margine ventrali 
maxime excurvato, postice et antice magis hiante, minus equi- 
valva, fossa cardinali et clavicula gracilioribus ; magna, tenui, 
ventraliter arcuata, strits incrementi gracillimis ; epidermide tenu- 
issima, vir antice rugosa ; cicatricibus muscularibus et pallii mar- 
yini 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, inequilaterali, inequivalva; parte postica bre- 
_ vissima, rostrata, rostro curto, lato; tenuissima, diaphana, albida ; 
superficie striis incrementi tenuibus, lineas radiantes huc et illuc 
viz monstrante, granulis minimis induta, epidermide tenuissima ; 
umbonibus prominentibus ; marginibus ventralibus et dorsalibus - 
anticis subequaliter 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. 
Hab. 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. villosiusculee” 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 
