MOLLUSCA. 75 



not well founded. It is adopted by most conchologists, although rejected 

 by Linnjieus, Cuvier, and Blainville. 



This family contains some of the most beautiful forms and finely colored 

 species, both in tint and pattern, among bivalve shells. There are upwards 

 of 150 living species, and the fossil species are also numerous, and chieiiy 

 found in the tertiary strata. There have been about sixty tertiary species 

 named from the formations of the United States. Venus mercenaria is an 

 inhabitant of both coasts of the North Atlantic, and is used for food. In the 

 markets of Philadelphia it bears the name of clam^ and in Boston that of 

 cwahog. The colored margin of the shell was used by the aborigines in 

 the manufacture of their wampum. Cytherea dione {pi. 77, fig. 37) is 

 remarkable for its longitudinal sulcations, and the double rows of lon^ 

 spines posteriorly. 



Fam. 8. Crassatellidce. This ftimily is represented by the genus 

 Crassatella, the mollusc of which being unknown, its affinities are doubtful. 

 There are about twenty recent species known, and a considerable number 

 of fossil ones, chiefly tertiary, but also cretaceous. Some authors place the 

 genus Astarte (also called Crassina) here, but Deshayes thinks the mollusc 

 (which is unknown) has an affinity with Venus. 



Class 2. Gastropoda. 



This class includes most mollusca with univalve shells, whether spiral or 

 not, as well as species without a shell. The head, absent in tlie Acephala, is 

 here present ; and on its presence Blainville's appellation of Paracephalophora 

 is founded, a character which is of more importance than the foot. 



Order 1. Polythalamia. This, the first systematic name applied to these 

 animals, was proposed by Soldani, 1789. More recently they have been 

 studied by D'Orbigny, who is the chief authority upon them, and by 

 Dujardin. The original name is defective, and both these authors have 

 conferred French names upon them, in contempt of those rules which keej) 

 nomenclature pure and uniform, names which are of no more account than 

 the German name Bauchfiiszler instead of gasteropoda / and should the 

 systematic name be adopted subsequently to such a vernacular one, and be 

 a translation of it, the author of the latter cannot be quoted for the sys- 

 tematic name. 



Tliese animals have been also named Foraminifera and Phizophoda. 

 Their classification is difficult. Their shell bears a distant resemblance to 

 that of certain cephalopoda, and on tiiis account they were for a considerable; 

 period referred to this class. D'Orbigny considers them as a distinct class 

 between the Echinodermata and Zoophyta, and Dujardin regards tiiem as 

 acalephae, and as allied to infusorial forms like Amiba and Difflugia. Agassis 

 regards them as the lowest form of the gasteropodous molhisca, and wo 

 place them provisionally here, although they seem to have neither head nor 

 foot, two important organs in this class. The apparent want of viscera 

 indicates a position below that of the Bryozoa, and although the locomotive 

 organs may be assumed as giving them a higher position, these are probably 

 merely a modification of the tentacles. 



