FREE INSTITUTE OF SCIENCE 



1007 



TERTIARY FAUNA OF FLORIDA ' 



The distant laterals, as in Cardium and Lucina, arise independently and 

 later. They are often not functional and naturally become obscure and some- 

 times obsolete. It is often difficult to say whether the projecting callosity at 

 the distal end of a nymph should be regarded as a lateral or not, and it often 

 happens in inequivalve species that the functions of a lateral lamina are per- 

 formed by an evenly callous projecting portion of the valve margin, not 

 differentiated into a recognizable lamina. I have regarded such hinge margins 

 as not constituting laminse in making up my formulae. It often happens that 

 the non-functional laminae are reduced to very small dimensions only visible 

 on close study and easily overlooked, whence such cases arise as in the genus 

 Metis, of which some species still retain ( as in M. alta Conr. ) a minute obsolete 

 lateral under the nymph, while others {M. interstriata Say) have entirely lost 

 it. I have not found in the literature a single case where the diagnostic char- 

 acters given for the various groups are uniformly correct in describing 

 the hinges. The teeth are subject to differences correlated with age. To 

 obtain an accurate idea of the cardinals it is often necessary to examine speci- 

 mens in the adolescent stages, as the teeth become in some cases crude and 

 irregular in adults, besides suffering from erosion. The bifid teeth are not, as 

 Noetling has apparently assumed, due to the coalescence of two originally 

 distinct lamellae, but the accretions to the teeth, being deposited by distinct 

 proliferations of the dorsal mantle-border, are naturally less profuse along 

 the line where two adjacent proliferations meet each other, and a groove re- 

 sults. The lateral laminae, on the other hand, are usually better developed in 

 the older individuals and sometimes wanting in the young. 



The ligament varies from extremely long and narrow, as in Phylloda, to 

 short and high, as in some species of Angnlus. The nymphs are usually larger 

 and more prominent in thin shells with short ligaments; subcircular species 

 always have a short ligament. The resilium is usually enclosed in the hemi- 

 cylindric ligament. In some forms, however, as Metis and Tellidora, the 

 resilium is much shorter than the ligament and evinces a tendency to become 

 internal, as in the Semelidce. In such instances it is notably thicker, espe- 

 cially towards the beaks. There is in a few forms, like Macalia, a tendency 

 towards an amphidetic area, and over this is frequently concentrated a certain 

 amount of dark periostracum, presenting an appearance as if the ligament 

 proper extended in front of the beaks. Mr. E. A. Smith called attention to 

 this in his report on the Challenger Pelecypoda, instancing T. donacina as an 

 example. I have not, however, been able to satisfy myself in any instance 

 that any portion of the true ligament extends forward of the beaks in Tellma, 



