1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 143 



The islands coustitiitiug the Tres Marias group are known as the 

 Maria Madre, the Sau Juanita, the Maria Magdalene, and the Maria 

 Cleofas, the Maria Madre being the largest. They are situated due 

 west of San Bias, and " consist of stratified rocks," separated from the 

 mainland " by a flat of not more than * thirty fathoms" in depth. 



All or nearly all of the forms credited to this group are from Maria 

 Madre; or if collected uj)Ou the others, the fact has not been stated 

 by Fisher, or the other collectors as far as I am aware. 



Of Socorro, the principal island of the more distant Revilla-Gigedo 

 group, we know but little or nothing. It was visited several years ago 

 by Grayson, the ornithologist. The few marine shells that have been 

 brought from there, are as would be supposed, familiar gulf forms. Of 

 the land shells nothing is known. These islands are situated in com- 

 paratively deep water, according to the Albatross, soundings from 1,500 

 to 1,800 fathoms. 



Class l^KLECYPODA. 

 Family O s i' \i e i d a^.. 



OSTREA PALMIII.A, C a r p e. n t « r. 



Rare (No. 74809, IT. S. N. M.). Pond's Ishmd southern jx.int of 

 Angeles Island, Gulf of California, Fisher; La Paz A, Forrer. 



This s])ecies is number 357 in Carpenter's Check-list of the Smitli- 

 sonian Institution, and in the ''Mazatlan JVFollusca," number "211 b: 

 Ostru'a '? ? amchaphlhi, var. palmiila.''^ 



Judging from Carpenters description in his Mazatlan shells, the 

 Fisher and Forrer examples belong to his species. The specimen before 

 me is mucli larger than the measurement given by Carpenter, and the 

 l)roportions are not tlie same; as oysters vary so much in this respect, 

 the individuals of one colony compared with the individuals of another, 

 this discrepancy may be allowed to pass without further comment. 



The chief example (Fisher's) measures 3-50 by 3-48 inches, a fine 

 specimen. 



The Forrer specimen which is a thick, solid shell shows 2 inches in 

 greatest length by 1|- inches in width. 



Carpenter gives the proportions and size of his type, as "long. 2-3, 

 lat. 1-0 [heightV' He says "remarkable for the palmated foliations on 

 the outer margin, * * * and for the row of denticles within this 

 limb and witliin tlie nacreous border, fitting into corresponding depres- 

 sions in the other valve." The palmated foliations radiate from just 

 beneath the cartilage in the under or lower valve to the margin. In 

 the upper valve these radiating ribs commence at a point about one- 

 third of the distance from the umbo; perhaps in a large number of 

 examples, some might exhibit this character as extending to the umbos. 



*A. Aga88i;z. 



