349 



MACTRIDJ^:. 



More than once we have had occasion to remark on 

 the impossibility of arranging genera and tribes in order 

 of affinity, and at the same time of maintaining a conti- 

 nuous sequence in a single line. The family before us is 

 an instance ; for, whilst there can be no question that it 

 has close affinities with the Veneridce^ it presents so many 

 features which remind us of the 3Iyadce that we seem to 

 have separated it from the last-named tribe by a forced 

 and unnatural barrier when we marshalled the Tellens and 

 their allies in the interspace. Yet they too affijrd indica- 

 tions of near affinity — relations which the microscope 

 has confirmed ; for we find Dr. Carpenter describing the 

 texture of the shell of Mactra as resembhng that of the 

 Tellinidce,^ but presenting more distinct indications of or- 

 ganic structure, exhibiting an unquestionable cellular layer 

 though not definitely developed, on the external surface, 

 and an inner layer of elongated cells with distinct bound- 

 aries.* 



The Mactrida have variously shaped, often tumid, 

 shells, sometimes thick, sometimes thin and invested ex- 

 ternally with a strong epidermis ; most of them are 

 smooth, or merely striated across externally. The valves 

 are connected together by a hinge consisting of a forked 

 diverging tooth in one raised on a ligamental fulcrum, 

 lodged in a cavity, which is marginated, in the other, a con- 

 * Carpenter, loo. cit. p. 103: 



