203 



SYNOPSIS OF A REVIEW OF THE GENERA OF RECENT AND 

 TERTIARY MACTRID^ AND MESODESMATID.E. 



By W. H. Dall, Palaeontologist, TJ.S. Geol. Survey, and Curator, 

 Dept. of Mollusks, TJ.S. Nat. Museum. 



Read November 9th, 1894. 



Ik the course of some palaeontological work upon which 1 have been 

 engaged during the past year, it became necessary to review the 

 Mactracea, which were found to be in a good deal of confusion, both 

 as regards systematic relations and nomenclature. As a result I have 

 been obliged to examine all the accessible material, conchological and 

 anatomical, and contemplated offering to the Society a synopsis of my 

 conclusions. 



I soon found that, if I attempted to include diagnostic characters, 

 this synopsis would reach inconvenient bulk for the Society's 

 Proceedings, besides requiring an excessive amount of explanation. 

 The hinge of Mactra has been so superficially studied, for the most 

 part, that in order to avoid tedious periphrasis it became necessary 

 to classify and name its several parts and investigate their dynamic 

 relations. The Pelecypod hinge is an organic machine for the attain- 

 ment of certain physical results, and the course of its development 

 and modification is very closely connected with the stresses and strains 

 to which it, considered as a piece of mechanism, is subjected. The 

 questions involved are still further complicated in this group by the 

 descent of a portion of the primitive ligament through the hinge 

 margin, and its modification to form an internal " cartilage."' Every 

 stage of the process is beautifully illustrated in one or another genus 

 of the Mactracea, from that in which the resilium is barely detached 

 from its parent ligament (as in Spisula) ; or has developed a shelly 

 barrier between the two {^Mactra s.s.); or exemplifies the descent of 

 the ligament after the resilium, though not in the same cavity 

 ( Cyclomactra) ; or, finally, the total immersion of both parts and 

 their enclosure in a common receptacle {Mulinia and Rangia). The 

 process is not only illustrated in its various stages by existing 

 Mactroid animals, but the succession of the fossil forms, from the 

 Chalk to the present day, gives us what we may permissibly recognize 

 as a serial succession of changes along a common line of descent, in 



1 Since this substance is totally different from cartilage, properly so-called, and the 

 ligament is sometimes also internal, it seems desirable to substitute a new name for 

 the part in question. Its function being invariably that of separating the valves by 

 its expansional elasticity, I have decided to adopt for it the name rcsUium, while, for 

 the pit in which it is frequently seated, we may retain the term chondrophore already 

 widely used. 



