282 NUTTALLINA. 



This form has not been noticed by subsequent authors. It seems- 

 to agree with the preceding species in the dorsal and lateral ribs, 

 and also those upon the anterior valve ; but as the interior is 

 unknown, its reference to Nuttallina is provisional. 



Subgenus MIDDENDORFFIA Carpenter, 1882. 



Middendorfia CPR., MS. and in DALL, Proc. U. S. Nat. Mus.,. 

 1881, pp. 284, 287, type Chiton polii Phil, non Desh., (Jan. 20, 

 1882.) Dawsonia CPR. 1873, preoc., same type. Beania CPR. 

 MS., not of Johnstone. Beanella DALL, Proc. U. S. Nat. Mus. 

 1881, pp. 284, 287, type C. pseudorissoi CPR. ined., in Mus. Cuming 

 no. 51. 



Valves exposed, rather square and short, granose outside with 

 low ribs at lateral areas and on head-valve. Insertion plates rather 

 sharp, not notably rugose, the teeth more or less thickened outside at 

 the slit-edges; slits of head-valve corresponding in position to exter- 

 nal ribs. Tail valve small with mucro behind the center, the teeth 

 directed forward, eaves and sinus coarsely spongy. Girdle narrow, 

 compactly covered with a pavement of minute striated scales and 

 spinose at the edge. Gills extending forward over three-fourths the 

 length of the foot. Type Ch. cinereus Poli, not Linn. 



This subgenus agrees with Nuitallina in the markedly porous or 

 spongy texture of eaves and sinus, in the external sculpture, and in 

 the forms of the insertion-teeth, except that in Middendorffia these 

 are thickened at the slit-edges. It differs from Nuttallina in the 

 rather flattened scale-like, rather than spine-like girdle appendages 

 but the younger spines of Nutallina scabra, when examined under a 

 strong power are often seen to be striated, appearing like finely 

 fluted ivory pillars ; and in some specimens of the same species 

 there is a marked tendency to form a marginal fringe of longer 

 spines. The differences between the West American and Med- 

 iterranean forms are therefore not great, being confined to (a) the 

 girdle covering, which often varies as much in closely allied species, 

 such as Ischnochiton conspicuus and I.acrior, or Chcetopleuraperuvi- 

 ana and C. hennahi, and (6) the thickened tooth-edges, which 

 varies considerable in Middendorffia and fails as a generic charac- 

 ter in Ceratozona and some other groups. 



It should be noted that in Middendorffia the true structure and 

 especially the sculpturing of the girdle-scales often cannot be satis- 

 factorily observed without a compound microscope ; and a magnifi- 



