On the Form j Growth, and Construction of Shells. 27 



other bivalve inollusca, until they meet with suitable places for 

 attachment. 



They are found from low water to one hundred and twenty 

 fathoms, and probably even deeper, and are distributed almost 

 in every sea at the present day, whilst their range in geological 

 time is only equalled by such forms as the microscopic Fora- 

 ■minifera and Entomostraca ; species of fossil Lingular being met 

 with in our oldest Cambrian and Silurian formations. At 

 present only seventy living species have been met with, but 

 as they are for the most part inhabitants of deep water, no 

 doubt many more will be discovered, for good marine dredges 

 are a comparatively modern invention, and deep-sea dredging 

 is no easy task, as the writer can testify from personal experi- 

 ence on the coast of Spain. 



More than one thousand extinct species of Brachiopoda have 

 been described, representing, of course, a succession of geo- 

 logical periods, so that it is not improbable that they are 

 nearly, if not quite, as numerous at the present day as formerly. 



Some of the fossil species attained a very large size, and 

 the paleozoic genera present remarkable variations in form 

 from each other, and also from any now living. Dr. Gustaf 

 Lindstrom has lately separated one group entirely, and placed 

 them in a separate order, to be called, Operculated Radiata of 

 the order Rugosa. 



Floating Shells. — The Pteropoda* or " wing- shells/ ' furnish 

 good illustrations of this form of molluscan life (see Coloured 

 Plate, Yol. x., p. 241, Fig. 1, Cleodora pyramidata) . li This little 

 group consists of animals whose entire life is passed in the 

 open sea, far away from any shelter, save what is afforded by 

 the floating Gulf Weed, and whose organization is specially 

 adapted to that sphere of existence. In appearance and habits 

 they strikingly resemble the fry of the ordinary sea-snails, 

 swimming, like them, by the vigorous flapping of a pair of 

 fins. To the naturalist ashore they are almost unknown, but 

 the voyager on the great ocean meets with them where there 

 is little else to arrest his attention, and marvels at their delicate 

 forms and almost incredible numbers. They swarm in the 

 tropics, and no less in Arctic seas, where by their myriads the 

 water is discoloured for leagues (Scoreshy). They are seen 

 swimming on the surface in the heat of the day, as well as in 

 the cool of the evening. Some of the larger" kinds have 

 prehensile tentacles, and their mouths armed with lingual 

 teeth ; so that, fragile as they are, they probably feed upon 

 still smaller and feebler creatures {e.g., Entomostraca). In 

 high latitudes they are the principal food of the whale, and of 

 many sea-birds. Their shells are rarely drifted on shore, but 



* So called from ■m(pov i a wing, and irovo; iro8o<r, a foot. 



