ARCA. 



know to be affixed there is an open space in the ventral 

 part of the valves (when closed) through which such 

 byssus passes, while in the inequivalve Arcse, and in some 

 others there is no such open space any more than there is 

 in CuculliBa. Moreover Lamarck informs us that some of 

 the Arcce live in the sand in the same manner as the Cardki, 

 so that it appears to us that the Arcacea, including Cu~ 

 cullcea are on one side related to Mytilus and on the other 

 to Cardium: and it might not be improper to divide the 

 Arcae into two or more genera, according as they are 

 either affixed or free, equi valve or inequivalve; neverthe- 

 less as there is a peculiarity in which they all agree, namely, 

 the straight hinge line with numerous small teeth, we 

 prefer for the present to keep them altogether. 



In general form the Arcae for the most part, when 

 placed with the ventral part downwards, may be not 

 unaptly compared to a little ship, whence their name ; the 

 area of the ligament forming the deck; they are equivalve 

 or nearly so; generally inequilateral, and mostly angular 

 at both dorsal ends^ though there are some species which 

 are rounded; and they are more or less ventricose, some 

 being remarkably so : the umbones are generally distant, 

 being separated by the area to which the external ligament 

 is attached and which it generally nearly covers: hinge 

 linear, straight, furnished with numerous very small teeth 

 on both sides. Muscular impressions two, lateral, distant. 



The Arcae abound on the coasts of nearly all countries, 

 and apparently in all climates ; several species are found 

 on our own coast. In a fossil state they are numerous in 

 the tertiary beds, and particular species are found even 

 as low as the Inferior Oolite. 



In our plate are represented, Fig. 1. The Area Noai, 

 one of those species which are attached by a strong ten- 

 dinous byssus : Fig. 2. Area rhombea, and Fig. Area 

 inaequivalvis. 



