456 GEOGRAPHICAL DISTRIBUTION OF STROM BUS. 



influence of its own in governing the distribution of life, at least in 

 the littoral zone, and it is well known that genera like litorina, 

 patella, purpura, fissurella, and trochus are found on rocky coasts 

 cardium, tdlina, solen, cerithium, on sands ; while mytilus is a lover 

 of gravel, and mi/a, lutraria, pullastra, frequent muddy shores. It is 

 probable that mollusca, like fishes, migrate, for frequently large num- 

 bers of dead shells are dredged without a trace of living animals. 



Existing Distribution of Genera. Any one who would become 

 a palaeontologist must be familiar with the geographical distribution 

 of existing genera, and to this end should indicate as far as possible 

 upon blank maps the homes of the species of each genus as they are 

 examined. For, only when the geographical areas occupied by species 

 are known, and considered in relation to their several distinctive char- 

 acters, is it possible to estimate the geological circumstances to which 

 the distribution is due, or the antecedent specific characters from 

 which the attributes of living species have been evolved. The study 

 of genera may thus sometimes aid in discovering to us changes in the 

 distribution of land by which the species have become distributed 

 over the world. And often when the genera of a family are compared 

 together, and compared with their fossil representatives, the mode of 

 origin of the genera, as well as the migrations and modifications which 

 species have undergone, are suggested with great probability. We 

 may illustrate this aspect of palseontology by reference to the Gas- 

 teropod genus Strombus, which dates back to the Cretaceous period, 

 is imperfectly known from the Tertiary rocks, and is widely dis- 

 tributed in existing seas. This genus includes some large species in 

 the West Indies, such as are commonly carved for cameos, and a 

 group of small species distinctive of the Philippine Islands, besides 

 the ordinary widely-distributed types. Some species of Strombus ap- 

 proach towards the allied genus Pteroceras, and others towards the 

 allied genus Kostellaria ; and among the Lower and Middle Tertiary 

 rocks of Europe we find the blending of generic characters is more 

 marked, and species occur which show that some of the areas from 

 which Strombus and Rostellaria are now absent, were inhabited by 

 those genera in comparatively late geological periods of time. So 

 that we are compelled to believe that the uplifting of existing 

 land has divided the species, and gathered them in affiliated groups 

 in definite localities. The story of the Strombs, or any family, 

 would take too much space to tell in detail here, and we can only 

 indicate the general distribution of species. First : In the Red Sea 

 are Strombus gallus, S. fasciatus, S. Ruppelli, and S. lineatus. 

 Strombus tricornis is common to the Red Sea and the Antilles ; 

 Strombus elegans is common to the Red Sea and Philippine Islands ; 

 and S. gibberulus has a wide range from the Red Sea by the 

 East Indies, Philippine Islands, and Moluccas to the Society Islands. 

 On the coasts of India and Ceylon are found Strombus Lamarcldi 

 and S. SiUbaldii ; but S. fissurella and S. canarium range to the 

 Philippines, while S. plicatus, with the last-named species, range 

 from India to the Moluccas. The Philippine Islands yield Strom- 



