INSTITUTE OF SCIENCE, PHILADELPHIA. 435 



length 3.0 mm. ; sutural laminae thin, sharp, smooth, continuous with the 

 insertion-plates, interrupted (1.5 mm.) at the jugum,the edge nearest the ju gal 

 area minutely serrate ; insertion-plates distinct, with a single sharp, clear-cut 

 slit, the portion of the plate behind the slit with its edge minutely serrate, 

 bearing about si.K denticles, the portion in front of* the slit entire; below the 

 valve is simple, smooth, the jugum with an entire edge ; the mucronation is 

 strengthened underneath with a sort of triangular gusset of shell-substance. 



I have taken for granted that the head-valve kindly presented by Mr. 

 Crosby, and the central valve found by myself, belong to the same species. 

 They have the dorsal angle about the same, and there are no apparent incom- 

 patibilities between them. When we consider that both are silicious pseudo- 

 morphs, it is marvelous that they should show the details mentioned. In the 

 absence of the armature of the girdle and the tail-valve, it is impossible to 

 refer the species to its particular section of the genus, which may be Trachy- 

 radsia, or some other of the groups showing Radsioid tendencies in the 

 insertion-plates. 



The worn head-valve of a species apparently belonging to this genus was 

 found in sand from the Newer Miocene or Older Pliocene of the Great Dismal 

 Swamp in Virginia. It has no surface-sculpture except concentric lines of 

 growth, is wide and short, with strong insertion-plate showing eleven slits. It 

 is, however, too imperfect to name. 



Superfamily OPSICHITONIA. 



Family MOPALIID^. 



Genus ACANTHOCHITON Leach. 



Acanthochiton spiculosus Reeve. 



Chilon spiculosa Reeve, Mon. Chiton, pi. 9, fig:. 47, No. 47. 



Phakeliopleura {Acantlwchites) spiculosa Shiittl., Berner, Mitth. 1S53, p. 203. 



Fossil in the Pliocene of the Caloosahatchie River, Florida, Dall ; living 

 on the Gulf of Mexico from Cedar Keys southward to the West Indies, 

 Hemphill, Dall and others. 



A fragment of a central valve was obtained in the marl, which agrees 

 fairly well with the corresponding part of a valve of the above-mentioned 

 species. The specimen is too imperfect to make the identification certain, but 

 it is at least probable. 



Class SCAPHOPODA. 



Order SOLENOCONCHIA. 



Family DENTALIID/E. 



This is a very ancient group, and all of its genera yet existing in the 

 recent fauna appear to be represented in the American Tertiary. 



