THE SHELLS OF THE GULF OF CALIFORNIA. 201 



and Pleistocene of American authors,) in each of which we have 

 species represented still living in the same seas or in the Mediter- 

 ranean or Boreal districts. If the species is in mature vigor, it may 

 still be found widely diffused. If, on the other hand, it be dying out 

 in its general area, it may preserve a lingering existence in very 

 remote localities which once were connected. Thus the Orbitolites 

 of the Paris basin is still living in the East Indies, although now 

 unknown in European seas; while the common gulf weed of the 

 modern Atlantic is believed by Prof. Forbes to be a further develop- 

 ment of the very same plant which floated (as now) in huge masses 

 in the ancient ocean of the Eocene. 



When the tertiary fossils on each side of the Rocky mountains 

 shall have been thoroughly explored, when the age of these moun- 

 tains in the narrow isthmus shall be better understood, when the 

 deep waters of the Gulf of Mexico and the Pacific coast shall have 

 been well dredged, we may be in a position to speak with confidence 

 on the points of similarity and of contrast in the two oceans. At pres- 

 ent we can do little more than accumulate facts for future explanation. 



In the case of Crepidula aculeata, however, the perfect specimens 

 brought by Mr. Dyson from Honduras correspond so exactly with 

 those from Mazatlan that it is hardly possible to resist the impression 

 that they are identical. Specimens from South Africa, from Sydney, 

 (Australia,) and from the Pacific islands also present no marks of 

 specific distinction. It appears to be one of the ubiquitous species, 

 of which several are found in various genera, and some are known to 

 have existed far back in time. Of this number is Saxicava arctica, 

 which has been found in all the three epochs of the English crag; is 

 now flourishing in the boreal as well as the temperate regions of Eu- 

 rope and America; has been found in the China seas and in Australia, 

 (C. testo, Forbes,) and attains respectable dimensions in the cavities of 

 our Mazatlan Spondylus. The Crepidula not only undergoes the 

 changes of form from nearly flat to deeply arched, from obese to elon- 

 gate, which every observer of the common slipper-limpet of the At- 

 lantic (C. fornicata, abundant from the icy shores of the St. Lawrence 

 to the tropical water the Gulf of Mexico) knows to prevail in that 

 species; but in sculptino it may either be crowded with short spines 

 (C. echinus, Brod.;) or have a few radiating lines of longer spines witli 

 nodulous interstices (C. hystrix, Brod.;) or be covered with an irregu- 

 lar mass of spiny knobs (normal state;) or lose the spines altogether 

 in roughened striaa (smooth-water form;) or even become almost des- 

 titute of sculpture, like some northern specimens of the stunted va- 

 riety (C. Californica, Nuttall.) Through all these changes it is recog- 

 nized by its spiral stomatelloid growth, exemplifying a section of the 

 genus the extreme forms of which approach Trochita; and by it* 

 beautifully waved deck-margin, which resembles a •— -' — -*. The 

 pointed centre, as the shell increases in size, generally leaves a char- 

 acteristic line on the surface of the deck, passing up to the vertex. But 

 often the point is rounded off, and even degenerates into a broad wave. 

 In one specimen, co-ordinate with this degeneracy, a sharp angle was 

 abnormally formed on one of the sides, so as to give the margin the 



