xii LNVERTEBRATA OF MASSACHUSETTS. 



A conviction that the value of the work would be greatly en- 

 hanced by figures of such objects as have not been represented, 

 or only in works which can rarely be seen, has induced me to 

 avail myself of the permission given to add the desirable plates. 

 The figures have all been drawn by my own hand, from nature. 

 The engravings are not highly finished, but are sufficiently char- 

 acteristic. 



Having often been embarrassed by an uncertainty as to the 

 true pronunciation of scientific names, and finding a great diversity 

 of modes practised among naturalists, I have attempted to remove 

 the difficulty, and have affixed the proper accents to every name. 

 In this labor I have been greatly aided by the classic accuracy of 

 Mr. C. Folsom, the conductor of the University Press, Cain- 

 bridge. 



It is necessary to advert briefly, also, to the position in which 

 a shell should be held when comparing it with a description. The 

 shell is supposed to be examined in the same attitude which it 

 assumes when its animal is in motion. In the bivalve shells the 

 beaks are at the summit, usually over, and always near the hinge, 

 and always directed inwards and forwards ; opposite to the beaks 

 is the base ; the height of the shell will of course be a line perpen- 

 dicular from the beaks to the base, or more properly, a line par- 

 allel to this, where the greatest altitude is found. The anterior 

 end is usually shorter and more exactly rounded than the poste- 

 rior or hinder end, which is also higher and more gaping than the 

 anterior. When the palleal impression has a nook or sinus, this 

 always opens backwards. The breadth will measure the greatest 

 distance between the sides of the two valves. If we take a uni- 

 valve shell and apply its aperture to the table, we shall have nearly 

 the position in which the animal carries it. The point of the 

 spire will then be directed backwards and upwards, and the oppo- 

 site extremity will be the front. The terms front, anterior, and 

 posterior, as now used, correspond to the terms base, lower, and 

 upper, of most books. The latter terms I have occasionally em- 

 ployed, when I thought they would convey most definite ideas ; 

 but they are less proper, and are always to be considered as sy- 

 nonymous with the former. Imagining the shell, then, to be car- 



