349 



MACTRIDi?^:. 



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 Vetieridce, it presents so many 

 features which remind us of the Myadce. 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 afford 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 

 TellinldfR^ 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, loc. dt, p. 103: 



