AMPHINEURA 487 



Key to the families of MesoplacopJiora here described : 



«i Tegmenta of the valves fully exposed. 

 bi Last valve without a posterior sinus 1, Ischngciiitonidae 



62 Last valve with a posterior sinus 2. Mopaliidae 



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



3. ACANTHGCHITONIDAE 



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 IschnocJiitonidae here described : 



ffj No radiating ribs on the anterior valve. 

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



63 Girdle not smooth. 



Ci Girdle hairy 1. Ch^topleura 



Cj Girdle granulated 3. Trachydermon 



C3 Girdle covered wath scales 4. Ischnochiton 



ffj Anterior valve with radiating ribs 5. Nuttallina 



Fig. 748 Fig. 749 Fig. 750 



Fig. 748 — Chwtopleura apiculata (Gould). Fig. 749 — Tonicella mannorea (Gould). 

 Fig. 750 — Trachydermon ruber (Gould). 



1. CHiETOPLETJRA Shuttleworth. Umbo of posterior valve central 

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



C. 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 (Fabrieius) (Fig. 740). Shell oval, buff-coloied, 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. 



