434 TRANSACTIONS OF THE WAGNER FREE 



and reinforced beliind by a transversely striated, low ridge of callus, behind 

 which again the jugal area is radiately striated. The tail-plate is somewhat 

 variable in form (as in recent Chitons) ; an average specimen measures 8.5 mm. 

 wide by 5.0 mm. long; the sculpture is of the same character as that on the 

 other valves, smooth or punctate on the mucro, radial in front of the mucro, 

 and longitudinal on the lateral areas ; there is sometimes a dimple at the centre 

 of radiation under the mucro, and the prominence of the apex varies consider- 

 ably ; the anterior laminse are connected by a narrow bridge across the jugal 

 region ; the insertion-plate resembles that of the head-valve, and has about 

 thirty fimbriations, many of them double. 



I have been thus detailed in describing this species since there is no 

 description or figure by which its systematic place might be determined. Con- 

 rad's figures, copied by De Gregorio, merely indicate the form and sculpture 

 of the upper surface of the tail-valve. De Gregorio's figure of the head-valve 

 is made from a poor and broken specimen. 



I have selected the name of eocenensis, of the two simultaneously applied 

 to this species, as more characteristic and based on a perfect specimen ; C. 

 antiqnus is extremely modern compared with the paleozoic species, and was 

 described from a worn valve. The form here described is identified from 

 specimens sent to the Smithsonian by Conrad many years ago. 



I have suggested a subgeneric name for this species, since I find its char- 

 acters not corresponding to the assemblage presented by any of the recent 

 forms, though evidently allied to the restricted genus Cliiton and to several 

 of its sections. No distinct slits are observable in the terminal plates, they are 

 fimbriated, but, with the exception of the denticular processes which consti- 

 tute the fringe, cannot be said to have any insertion-plates in the proper 

 sense of the word. 



Genus ISOHNOOHITON Gray. 

 Ischnochiton tampaensis 11. s. 

 Plate 23, figure 23. 

 - Older Miocene of the Orthaulax bed at Ballast Point, Tampa Bay, 

 Florida, Dall and Cro.sby. 



Head-plate elevated, about 4 mm. wide and ^^.^ mm. long, the upper sur- 

 face convex, simple, with fine obscure radial granulations ;' toward the periph- 

 ery the lines of growth are somewhat impressed, giving rise to a few concen- 

 tric ridges; eaves well-marked, insertion-plate thin, sharp, with eleven narrow, 

 clean-cut slits. 



Central valves with an angle of about 100°, the surface minutely granular 

 and with microscopic pittings ; central area sculptured with (on each side 

 about five) sparse, distant, longitudinal, chain-like threads, somewhat irregu- 

 larly disposed ; lateral areas obscure, faintly marked by one or two obsolete 

 radii, or none ; dorsal ridge rounded, submucronate ; width of valve 6.5 ; 



