TRANSACTIONS OF WAGNER 

 602 



TERTIARY FAUNA OF FLORIDA 



free inner edge of the cardinal plate has disappeared, so that the edge, as it 

 now is, is close to the inner ends of the transverse teeth. The character of 

 the lateral tooth remains much the same as in typical Pleurodon. 



Cyrilla mitnita Cpr., from thirty fathoms off Catalina Island, California, 

 shows a further step in modification. The fossette has become still larger, 

 none of the anterior cardinal teeth is left. The four posterior cardinals are 

 . of the bent or V-shaped variety, and the cardinal plate and shell have become 

 more solid and heavy, though the shell is smaller than that of C. sulcata. The 

 ligament is still wholly internal and the cardinal plate solid and flat. The 

 wing-like expansions of its outer margin, so notable in all the species of Plcn- 

 rodon, are gone. The character of the shell in both sections of the genus is 

 the same, that is, glassy or Leda-like, not nacreous. 



In the preceding discussion of the group it will be observed that the 

 differences exhibit a rather gradual modification from some such form as 

 Nucula or Letia. These small shells, as in the case of the Gastropod Liotice, 

 show that the line between nacreous and non-nacreous shells is very faintly 

 marked at times, and the difference, in such cases, cannot have much sys- 

 tematic weight. The group in question is composed of small shells, as their 

 relative measurements will show. Thus, in longest diameter we have Plcn- 

 rodon ova/is 1.75, P. miliaris 3, P. ScgucnzcE 5, P. Woodii 2.75, P. Adamsi 3.25, 

 Cyrilla sulcata 2.25, and Cyrilla mil nit a 2.12 mm. 



The corresponding shorter anteroposterior diameter is for P. ovalis i, P. 

 miliaris 2, P. Scgucnzce 4, P. Woodii 1.75, /'. Adainsi 2.87, Cyrilla sulcata 1.87, 

 and Cyrilla munita 1.75 mm. 



The form regarded by Mr. Smith as a variety of P. ovalis Wood, in a 

 recent state, from the Cape of Good Hope, measures 3.5 mm. high by 2.5 

 long and 1.75 in transverse diameter. The figure does not show the teeth 

 with sufficient clearness to determine its relations, but knowing the great 

 difficulty of seeing and then representing so minute and complicated a struc- 

 ture, this is not to be wondered at. I have a doubt as to the identity of this 

 species with the Crag fossil, but Mr. Smith's judgment in such a matter is 

 not to be lightly set aside. In this discussion the side of the hinge bearino- 



o o 



the lateral teeth has been treated as posterior, thus making the ligament an- 

 terior, as in Citspidaria, but I am by no means satisfied that this is the correct 

 view, and should not be surprised if it should be found necessary to reverse 

 these terms when a living specimen shall have been examined. For those 

 who object to the name Pleurodon on account of the existence of the prior 



