CALLISTOPLACIN^E. 259 



acute teeth. Eaves of posterior valve wide, flat, subspongy; of 

 other valves moderate, hardly grooved. Sinus narrow, deep, hardly~ 

 toothed, slit at the sides, at the junction of the sutural plates. 



Girdle leathery, smooth or most minutely and closely downy. 



Length 29, breadth 16* mill. ; divergence 110-120. 



Habitat unknown. 



Arthuria filosa CPU. MS. and in DALL, Proc. U. S. Nat. Mus. 1881, 

 .p. 287 (name only). Types in Mus. Cuming, nos. 23, 38. 



Carpenter's description is given above. He remarks : 



" This shell has the tail plate of Nuttallina, but the other valves 

 more resemble Chcetopleura or Tonicella. It is known externally from 

 Nuttallina by the nearly smooth girdle ; but would hardly be distin- 

 guished from Ornithochiton except by the regular articulation of the 

 insertion-plates. In the Ischnoid genera, the posterior profile from 

 the mucro is often concave ; in this the anterior. In consequence of 

 the great projection of the beak, both in Nuttallina and Arthuria, 

 the posterior teeth appear plumulate rather than fissured ; a char- 

 acter also seen in Callistochiton. The terminal valve in both the 

 specimens examined are exactly alike in color as well as markings; 

 but the central valves in one specimen are of a much lighter color, 

 with more delicate and well developed sculpture." 



Subfamily CALLISTOPLACIIOS. 



Ischnochitonidse in which the slits of the anterior valve correspond 

 in number and position to the radial ribs of the exterior. Teeth 

 generally thickened at the edges of the slits. 



In this subfamily, which seems to be a natural division, may be 

 placed a number of genera widely scattered in Carpenter's scheme. 

 These genera fall into three groups, or branches, lettered a, aa, and 

 aaa in the following table. The first of them seems to have affinities 

 with the typical Ischnochitons, the second slightly resembles the 

 Acanthochitidce, and the third may be somewhat allied to the 

 Mopaliidce. 



The number of anterior slits sometimes exceeds the number of 

 external ribs, so that the subfamily diagnosis must not be taken too 

 literally. The plan of structure is sufficiently obvious; but 

 exuberant Nature knows nothing of absolute adherence to rules. 



