INSTITUTE OF SCIENCE, PHILADELPHIA. 433 



The best-known species is Chiton eocciicnsis Conrad, from the Ciaibornian, 

 which has several synonyms. A single species, C. transenna, is described by 

 H. C. Lea from the Chesapeake Miocene of Petersburg, Va. His description 

 and figure are insufficient to enable one to refer the species to its proper place 

 in the family. To these I am now able to add one species from the Older 

 Miocene and one from the Pliocene formation. 



Family CHITONID.E. 



Genus CHITON Linne. 



Type Chiton tubcrculatus Linn6. 



Subgenus Trachyodon Dall. 



Insertion-plates broad, strong, divided by deep grooves into numerous 



bifid "teeth," or fimbriae, in the head and tail-plates; central valves with the 



plates continuous, thin and flat from the jugum to the girdle, where they have 



a single slit, on each side of which the plate is strong and fimbriated ; sculpture 



marked, with the lateral areas distinct, the tail-plate with a prominent mucro. 



Type, C. eoceneiisis Conrad. 



Chiton (Trachyodon) eocenensis Conrad. 



Cliiton eocenensis Conrad, Proc. Acad. Nat. Sci. Phila. vii. p. 266, 1855 ; Am. Journ. Concli. 

 i. p. 212, pi. XX. fig. 6, 1865 ; De Gregorio, Men. Claib., p. 170, pi. 16, fig. 39, 1S90 (tail- 

 valve). 



Oiiton aniiquus Conrad, Morton, Syn. App., p. 6, 1S34 (name only) ; Proc. Acad. Nat. Sci. 

 Phila. vii. p. 266, 1S55 ; Am. Journ. Conch, i. p. 2:2, pi. xx. fig. 7, 1S65 ; De Gregorio, 

 op. cit. p. 170, pi. 16, fig. 38, 1890 (tail-valve). 



Chiton? prostr emus De Gregorio, op. cit. p. 170, pi. 16, figs. 40-42, 1S90 (head-valve). 



Eocene of the Claiborne sands at Claiborne, Ala., Conrad, Johnson, 

 Burns, etc. 



The species is quite wide in front, and narrows toward the tail — at least so 

 far as the exposed surface of the valves was concerned, since the widest tail- 

 valve does not equal the width of the average head-valve in a considerable 

 number of specimens. The central valves are very short and wide ; the 

 central area on each side the jugum is longitudinally sculptured with about 

 twenty-five coarse, elevated, sometimes dichotomous, rounded ridges with 

 narrower interspaces ; I have not seen the jugal portion ; the lateral areas are 

 sculptured with six or seven radiating ridges otherwise like those of the 

 central area ; the lateral part of the insertion-plate has six or more finibria- 

 tions on each side of the notch ; the head-plate is short and very wide (3.6 x 

 I i.o mm.), the median portion in advance of the outer ends, slightly upturned, 

 but not mucronate ; the sculpture consists of about thirty dichotomous radii, 

 obscurely nodulous, or vermicular, and separated by narrower interspaces ; the 

 eaves are thin and very narrow ; when the surface is worn the nodular promi- 

 nences give place to punctations (C antiquus) ; insertion-plate exhibiting long, 

 strong paired ridges, corresponding roughly to the ribs of the upper surface 



