278 NUTTALLINA. 



fornia. Nuttallina is divisible into two closely allied sections or sub- 

 genera, as follows : 



Subgenus NUTTALLINA s. sir. Side-slits two in each valve, the pos- 

 terior one generally subobsolete, but indicated by a porous slit- 

 ray. Teeth hardly or not thickened at the edges of the slits; 

 girdle with minute, often chaffy spinelets and longer round spines. 



Subgenus MIDDENDORFFIA Cpr. Side-slits single. Teeth more or 

 less thickened at the edges; girdle clothed with minute striated 

 flattened scales and having a marginal row of flat striated 

 bristles. 



Subgenus NUTTALLINA Carpenter (restricted.) 



Nuttallina CPR., Table of Reg. Chitons, 1873. DALL, (Amer. 

 Journ. Conch, vii, p. 134), Proc. U. S. Nat. Mus. 1878, p. 333 

 (Feb. 14, 1879.) 



Valves exposed, granulated, having long smooth sharp teeth ; the 

 slits of the median valves obsoletely doubled, those of the anterior 

 valve corresponding in position to the external ribs, the teeth not 

 thickened at the edges of the slits ; mucro of posterior valve poste- 

 rior, terminal but not marginal ; teeth of posterior valve short, 

 chisel-shaped, directed forward. Eaves and sinus spongy. Girdle 

 bearing short, rigid spines. Gills extending the whole length of the 

 foot. Type N. seabra Cpr. Ch. californicus (Nutt.) Rve. 



The shell in this genus resembles that of typical Acanthopleura 

 (A. spinosa Brug.) in the elongated valves, double slitting of the 

 median valves, and the spinose girdle. It differs in the smoothness 

 of the sharp teeth and the spongy sinus, characters of much greater 

 importance than those first enumerated. The likeness of Nuttallina 

 to Acanthopleura is, in fact, merely one of analogy, having no mean- 

 ing deeper than that story of the development of different stocks 

 along parallel lines, repeated so often in the history of Chitons. In 

 its fundamental characters, Nuttallina agrees with Ceratozona; but 

 the more shelly texture of the girdle-spines, the spongy sinus, 

 and propless teeth afford sufficient ground for generic distinction. 



Authors and collectors have generally recognized but one Cali- 

 fornia species ; but there seem to be two. It is useless to try to 

 study them without separating the valves, as the specific characters 

 are not very apparent in specimens preserved in the ordinary- 

 manner. 



