214 MOLLUSCA FROM THE CRAG. 



Artemis,* Poll. 1791. 



Chama (Dosin.) Adans., \7a7. 



DosiNiA. Scopoli, \777 ; Gray, 1851. 



CiRCOMPHALUS (sp.) Klein, 1/53. 



Artemis ET Artemiderma. Poll, 1791. 



Orbiculus. Megerle, 1811. 



Asa. Leach, MSS., 1819, fide De/rance. 



ExoLETA. Brown, 1827. 



ARCTOe. Risso, 1826. 



Taras. Id. 



Arthemis. Desk., Phil., Blainv., Agass. 



Gen. Char. Shell suborbicular or lenticular, equivalved, more or less compressed, 

 generally strong and closed, surface covered with concentric striae ; margins entire. 

 Hinge composed of three cardinal teeth in one valve, and four in the other; no 

 lateral teeth. Ligament external, with a strongly defined, and generally deep lunule. 

 Impressions of the adductor muscles ovate, nearly equal, that by the mantle deeply 

 sinuated. 



Animal suborbicular, with the edges of the mantle mostly disconnected ; margins 

 entire, or partially serrated. Siphonal tubes long, united to their extremities. Foot 

 large, semilunar, or crescent-shaped, exsertible at the middle or basal margin of the 

 valves. 



This is a well-marked genus, said to be characterised especially by its animal 

 inhabitant, but if our reliance were solely upon the softer parts, to the entire exclusion 

 of the shell, it might be difficult to know where it should be placed. There is, however, 

 not much doubt respecting its relationship to the Venus group, but this affinity is better 

 displayed in the hard or shdly covering of the animal, than by any general resemblance 

 of either the foot, or the siphons of the mantle. The long siphonal tubes of this genus 

 are indicated in the shell by the impression of the retractor muscles being placed far in the 

 interior, so as to give what is called a very deep palleal scar or sinus, denoting, by its 

 depth, a corresponding proportion in the length of the tubes, this however, is not an 

 infallible criterion as to their length, nor does it appear to be a character of any great 

 family dependence, for in Lucinopsis the interior of the shell is marked by the impression 

 of the mantle with quite as deep a sinus, while the animal has the tubes short and slightly 

 exsertile, with a slender and delicate foot, and has little or no resemblance, any further 

 than in the form of the mantle mark, to animals of this genus, where the siphons are 

 long and united, and the foot is broad and splayed. 



In the recent state it has a wide geographical range ; as a fossil I am not aware of 

 its having been found in any older Formation than the Middle Tertiaries. 



* Etym., a proper name. 



