CHITONID^. 343 



CHITON, Lam. Girdle covered with distinct scales ; anterior 

 and posterior valves with many slits, middle valve with one ; 

 teeth blunt, serrate ; eaves short, spongy ; sinus squared, den- 

 ticulate ; gills ambient. 



RADSIA, Gray. Teeth in middle valve two or more ; difters 

 also from Chiton in having side-slits. G. Barnesii. 



TONiciA, Gray. Anterior and posterior valves with many slits, 

 middle with one; teeth sharp, serrate; eaves short, spong}'^; 

 sinus squared, denticulate ; girdle smooth or downy ; gills en- 

 circling. G. elegans^ Fremb. (Ixxxv, 80). 



PANNETTiA, Dall. lusertiou-plates regular, branching ; middle 

 valve with one, anterior and posterior with nine slits ; teeth sharp, 

 serrate, long; eaves very short, sinus squared, denticulate; 

 girdle smooth, spreading ; gills encircling. 



EUDOXOCHiTON, Shutt. Insertiou-plates unfissured ; teeth blunt, 

 fimbriate; eaves short; sinus small, laminse united; girdle hair3^ 



GRASPEDOCHiTON, Shutt. lusertion-platcs regular ; middle valve 

 with one slit, anterior with live, posterior with. eight ; girdle very 

 minutely asperulate. The posterior valve is fimbriated in the 

 middle. A doubtful group. 



D. Acanthoidea. 

 Insertion-plates thrown forward. 



* Plates broad, pectinated. 

 scLEROCHiTON, Cpr. Zone as in Bnoplochiton ; laminae obtuse, 



pectinate ; sinus waved, smooth. Most like Bnoplochiton, from 

 which it difi'ers in the articulation of the tail-plate and the sub- 

 central mucro. 



** Plates sharp, grooved outside. 



ACANTHOPLEURA, Guilding. (Lucia, Gould.) Anterior and 

 posterior valves with many slits, middle with one ; teeth blunt, 

 grooved; eaves projecting, grooved ; sinus lai-ge, waved ; girdle 

 with calcareous bristles. G. spiniger, Sowb. (Ixxxvi, 94). 



FRANCisiA, Carp. Valves partly immersed, planate ; central 

 laminse with several slits ; sinus lobed. Bears the same relation 

 to Acanthopleura that Fannettia does to Tonicia, with the 

 additional peculiarity of Radsioid nicks in the central valves. 

 G. spinosus, Brug. (Ixxxv, 81). 



* * * Plates sharp, smooth. 



DiNOPLAX, Carpenter. Armor heavy, alate ; mucro not ele- 

 vated, submedian ; laminse separate, acute, smooth; last valve 

 extending forwards ; sinus small ; girdle coriaceous, spinulose 

 in fascicles. G. gigas, Linn. (Ixxxv, 88). 



MiDDENDORFiA, Cpr. (Dawsonia, Cpr.) Armor and girdle as 

 in Acanthopleura ; laminse acute, rugose exteriorly ; the sinus 

 planate, not laminated. G. Polii, Phil. 



