THE SHELLS OF THE GULF OF CALIFORNIA. 197 



and algae, which are yet awaiting the attention of naturalists acquainted 

 with those special departments. We propose first to examine the 

 creatures which make their abode on the outside of these oyster 

 valves. 



Certain smooth, oval spaces bear testimony to the former presence 

 of many kinds of limpets. Some of those creatures, (as e.g. the Pa- 

 tella Mexicana, or giant limpet, which is sometimes a foot in length 

 and large enough for a basin) prefer to live on the rocks ; others are 

 always found on dead shells ; others again always adhere to living- 

 ones. The "circulation of water caused by the breathing currents of 

 the larger animal is no doubt congenial to their tastes. Most mollusks 

 have the power not only of forming, but also ot absorbing shelly 

 matter ; and these limpets, by the constant action of their strong 

 muscular foot, eat into the shell of the spondylus and leave a mark by 

 which each species can generally be recognized. Some of them make 

 regular excursions to browse on the algse and nullipore which they 

 rasp off with their thousand-toothed lingual ribbon, always returning 

 to their own hole to sleep ; but others appear to lead a sedentary life, 

 depending, like the bivalves, on whatever nutriment the water brings 

 within their reach. These, which go by the common names of ''bon- 

 net," "slipper," or "cup-and-saucer" limpets, are more highly or- 

 ganized than their more active neighbors ; the gill being a delicate 

 little comb at the back of the neck, and the sexes being distinct. The 

 Calyptrasids ("slipper" and " cup-and-saucer " limpets) found on the 

 Spondylus valves are the most beautiful and varied that are known in 

 any part of the world. The shells are large and thin, delicately fur- 

 rowed, and as it were engine-turned with a profusion of tubercles, 

 which sometimes rise up into long hollow spines. The colors vary 

 from white to a rich black-brown, or are variously mottled with sienna, 

 while the shape may be either an elevated cone or a widely spreading 

 disk. Sometimes the same individual will begin with one form and 

 sculpture-pattern, and suddenly change to another ; others again seem 

 to develop permanent and widely differing varieties. Occasionally a 

 starved or diseased Mazatlanian will present the aspect which is 

 normal on the colder shores of South America; exchanging its thin 

 texture and delicate sculpture for a coarse, solid, and nearly smooth 

 shell. So far the views lately propounded with such ability by the 

 celebrated author of the "Voyage of the Beagle" meet with sufficient 

 confirmation ; and yet, amid all its changes, there is a habit of ' grotUth, 

 hard to describe and yet easily recognized by the practised eye, which 

 not only unites the most aberrant forms, but at once separates them 

 from neighboring species found on the same coast and appearing very 

 similar to the common observer. The ordinary plan of only preserv- 

 ing in collections a few picked specimens displaying marked pecu- 

 liarities, is by no means favorable to the elimination of truth in 

 reference to specific variation. These extreme forms are very natu- 

 rally described as. distinct species, the intermediate connecting links 

 not'passing before the view of the naturalist. On showing to a dis- 

 tinguished author a carefully eliminated suite of Mazatlan specimens 

 connecting the smooth, thin, flat Crepidula squama, Brad, with the 



