EXPLANATION OF TER3IS 399 



always spring from the bosses^ and, widening as they 

 approach the ventral margin, become radiated. 



Lunate, or lunulate, assuming the form of a crescent o. 

 half-moon. Tiie aperture of the nerits is of this de« 

 scription, as well as the muscular impressions of 

 most bivalves. 



Mamillary, or mamillated. Smooth, thick, and obtuse, 

 like a teat. The apex of the typical volutes, as 

 Voluta olla, Diadema &c. are strictly of this de- 

 scription, as also those of Harpula fulgetrum, Sea- 

 phella papulosa, and other volutes : but C. vesperiilio 

 is not so, for the spire is distinctly plaited^ and even 

 rough with obsolete spines. 



Measurement, see Length. 



Multivalve. The oiily multivalve Testacea, properly 

 so called, are the chitons and the perforating genera 

 belonging or closely allied to Teredo and Pholas. The 

 Cirripedes are annulose animals or insects_, and have 

 no place whatever among the molluscous tribes, any 

 more than beetles or butterflies. 



Muscular Impressio7is : those indented marks in ace- 

 phalous bivalves, indicating the insertion of those 

 muscles by which the animal is attached to its shell. 

 Of these there are three principal sorts, — lateral, 

 central, and paUial. 1 . The lateral impressions indicate 

 the animal to have two adductor muscles, as in Unio 

 Cardium, and aU the most typical bivalves. 2. A 

 central impression shows us the presence of but 

 one adductor muscle, which is generally in the middle 

 of the shell, as in the common oyster. AU these 

 shells, however, are furnished with the third or pal- 

 lial muscle, which is indicated by a depressed line, 

 often sinuated, running parallel with the ventral 

 margin. 



Obsolete. This term, in conchology, is more properly 

 restricted to implying a faint indication of par- 

 ticular characters. Thus, very slight and partially 



