280 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



The characters for the groups herein described are due to Dr. Car- 

 penter, and are given mostly in his own words. It will be observed 

 that the groups named by others are restricted by him, by elimination 

 of incongruous material included with the original types. 



Helminthochiton Salter. 



Helminthochiton Salter (pars) § 1, Proc. Geol. Soc, 1846, pp. 49, 51,52, fig. 6 (^$2, 3, and 

 figs. 2, 3, exclus.). 



Lorica leptoidea, elongata, regularis; mucro ischnoideus; valvEe ter- 

 minales haud sinuatse; apophyses °? (ignotse). 



Helminthochiton may be described as a greatly lengthened chitonous 

 animal with the valves thin and angular, and perhaps without apophyses, 

 though the negative evidence is not sufficient to establish so remarkable a, 

 departure from the type of the class in general. It differs from Grypho- 

 chiton in having the terminal valves regular, not sinuate, and in having 

 a regular subcentral Ischnoid mucro instead of the Gry2)ha'a-\ike beak 

 characteristic of the second section. It may be regarded as a Leptoid 

 Ischnoplax with the valves thrown forward. 



Species. 



Helminthochiton Griffithi Salter, 1. c., pp. 51, 52, fig. 6. Silurian of Ireland. (Type.) 

 Helminthochiton 2)riscoidcs, CuTimuteT. Devonian of Vilmar ; Schultze. (Mus. Comp. 

 Zoology.) 



Gryphochiton (Gray) Carpenter. 



Gruphochiton Gray (pars.), P. Z.'S., 1847, ji. 70; no diagnosis. 



Lorica regularis elongata; laminae laterales nulkne, suturales parvse, 

 a sinu simplici lato separatee ; mucro postice medianus, incurvatus; regio 

 capitis et caudoe valde sinuata. Type G. priscus Munster. 



Gryphochiton resembles a Leptochiton greatly drawn out and with the 

 terminal valves more sinuated than has been observed in any recent 

 Chiton. 



Species. 



Gryphochiton priscus MUnster, Beitr. zur petr. kuudo, 1, p. 38, fig. 4, 1839. Carbonifer- 

 ous of Tournay. 



Gryphochiton mempiscus Ryckholt, Bull. Acad. Roy. des Sci. de Bruxelles, 1845, p. 48, 

 no. 4, pi. 2, figs. 5, 6, 7, 8. Carboniferous of Tournay. 



Gryphochiton triangulatum Carpenter, Ryckbolt, 1. c., pi. 2, figs. 4, 9, 10. Same locality. 



Gryphochiton nervicamis, Ryckbolt, 1. c, p. 47, No. 3, pi. 1, figs. 7, 8, 9, lo45. Same 

 locality. 



Subgenus Chonechiton Carpenter. 



Lorica leptoidea, valvae centrales Gryphochitoni similes, projectaej 

 valva postica mucrone postico, infundibuliformi. 



This bears the same relation to Choneplax which Loricites does to 

 Lorica, i. e. similarity, except in the absence of laminse of insertion. 

 It may be described as a Leptoid Choneplax. In the recent shell the 



