THE ROCK SHELLS. 



FRANK COLLINS BAKER, 

 Curator of the Chicago Academy of Sciences. 



T 



HE rock shells or murices are 

 among the most beautiful and 

 interesting of all the mollusks 

 or shell fish, and are a favorite 

 among collectors. Their peculiar 

 spiny shells and brilliant colors caused 

 them to be among the first mollusks 

 studied by naturalists and we find 

 them, therefore, described in the 

 earliest works on natural history. 



There are about two hundred dif 

 ferent kinds of rock shells, mostly con 

 fined to the tropical and subtropical 

 seas, although a few are found in tem 

 perate climes. The greatest number 

 of these are found about rocks at low 

 water but not a few are inhabitants of 

 waters as deep as fifty fathoms or 

 more. In our own country they are 

 abundant along the coast of Panama, 

 the Gulf of California, Florida and the 

 islands of the West Indies, but the 

 largest number of varieties comes from 

 the Indian Ocean, Japan, the Philip 

 pines and Australia. The more 

 brightly colored varieties are from 

 tropical seas, while the dull, plain 

 species are from subtropical or tem 

 perate climes. 



The murices are peculiar in having 

 their shells ornamented by numerous 

 projections, which vary from long, 

 needle-like spines to simple fluted 

 frills. What these spines and frills are 

 for would probably puzzle the ordinary 

 observer, as they would seem at first 

 sight to be in the way. In some cases 

 they are simply ornamental, but in the 

 main they are protective and enable 

 the animal to escape being eaten by 

 some voracious fish. This is known as 

 protective adaptation and was probably 

 brought about in this manner: the 

 murices, or their ancestors, did not at 

 first have spiny shells, and they fell an 

 easy prey to the fishes. As time went 

 on a few individuals, through some 

 modification of environment, developed 

 small spines or prominences. The 

 animals having these were not eaten 



by fishes as the knobs and spines 

 caused the fishes pain when swallowed, 

 therefore they preferred the animals 

 with smoother shells. In time this 

 modification caused a weeding-out pro 

 cess, the animals with smoother shells 

 being exterminated and those with 

 spiny shells increasing in numbers and 

 becoming more spiny as one generation 

 succeeded another. This continued 

 until the present time and is going on 

 even now. 



Another interesting fact concerning 

 the development of this ornamentation 

 is that the smoother shells inhabit 

 rocky shores where the waves are con 

 stantly beating in with greater or lesser 

 violence, while the more spiny in 

 dividuals live in protected and com 

 paratively still water. This adds ad 

 ditional weight to the theory expressed 

 in the last paragraph, for the fish which 

 feed upon these shells do not, as a 

 rule, inhabit localities where the water 

 is rough, as along a rocky shore, but 

 live abundantly in protected bays and 

 lagoons in which the spiny murices are 

 found. 



There are shown on the plate eight 

 species of rock shells, all more or less 

 common. The first one for us to con 

 sider may be called Venus' Comb, 

 (Murex tribulus) and is found in China, 

 Japan and the Indian Ocean. It be 

 longs to a group of shells which is 

 characterized by a long snout or canal, 

 and long, pointed spines. The color is 

 yellowish; in one variety the spines 

 are tipped with black. 



A shell which is found on the mantel 

 in every household is known as the 

 Branched Rock Shell (Murex ramosus}, 

 which is widely distributed, being 

 found in the Red Sea, the Indian 

 Ocean, New Zealand, Australia and the 

 Central Pacific Ocean, and attains a 

 large size, some specimens reaching 

 the length of a foot and weighing 

 several pounds. The aperture is fre 

 quently tinged with a deep, beautiful 



191 



