AMPHINEUEA 487 



Key to the families of Mcsoplacopliora here described : 



! Tegmenta of the valves fully exposed. 



&! Last valve without a posterior sinus 1. ISCHNOCHITONIDAE 



& a Last valve with a posterior sinus 2. MOPALIIDAE 



% Tegmenta more or less reduced or covered by the girdle. 



3. ACANTHOCHITONIDAE 



FAMILY 1. ISCHNOCHITONIDAE. 



Valves of large size and exposed, the anterior and posterior ones 

 similarly articulated; insertion plates smooth or nearly so and slit into 

 teeth which project outwards: 12 genera, containing some of the common- 

 est species along our shores. 



Key to the genera of Ischnochitonidae here described: 



! No radiating ribs on the anterior valve. 



6i Girdle smooth, without scales or hairs 2. TONICELLA 



& a Girdle not smooth. 



Ci Girdle hairy 1. CPLETOPLEUBA 



c a Girdle granulated 3. TRACHYDERMON 



c 3 Girdle covered with scales 4. ISCHNOCHITON 



a a Anterior valve with radiating ribs 5. NUTTALLINA 





Fig. 748 Fig. 749 Fig. 750 



Fig. 748 Chcetopleura apiculata (Gould). Fig. 749 Tonicella nlarmorea (Gould). 

 Fig. 750 Trachydermon ruber (Gould). 



1. CHJETOPLETTRA Shuttleworth. Umbo of posterior valve central 

 or anterior; girdle leathery and more or less hairy: 16 species. 



0. apiculata (Say) (Fig. 748). Shell oval, light buff or ashen, some- 

 times reddish in southern specimens, keeled, central areas with longitu- 

 dinal rows and lateral areas and end valves with irregularly scattered 

 tubercles; gills, 24 on a side, extending almost to anterior end; length 

 17 mm. ; breadth 10 mm. : Cape Cod to Florida, very common in shallow 

 water. 



2. TONICELLA Carpenter. Umbo of posterior valve anterior; girdle 

 leathery and smooth, without scales or hairs: 7 species. 



T. marmorea (Fabricius) (Fig. 749). Shell oval, buff-colored, closely 

 speckled with dark red; surface densely, microscopically granulated; 

 gills 20 to 25 on a side, extending three-quarters the length of the foot; 

 length up to 40 mm. ; breadth 24 mm. : circumpolar, south to Massachu- 

 setts Bay and to Aleutian Islands; Europe. 



