Cephalopoda of the Cincinnati Group. 121 



described as orange chrome vary with the light, becuining gokleu 

 brown or greenish brown, according to the light ; there i.s also an in- 

 distinct tuft of brown scales on the dorsal margin, about the mitldle. 

 I have not seen B. trifasciella, Clem. It must resemble this species 

 closely, but I do not find the third silvery streak mentioned by Dr. 

 Clemens. 



Class GephaJopoda (Cuvier), as represented in the Cincinnati Group. 



By S. A. Miller. 



This class Is divided into two orders: 1st, Dihrmirhkiia (two gilled); 

 2d, Tdmhranvhlata (four gilled). The Dibramhlata are the highest 

 organisms in the molluscan scale, and are extensively represented, by 

 numerous families, in the seas, at the present time, but are not known 

 to have existed during the Silurian age The Tdrabranchiata are 

 represented by only one living ftimily, the Nauti/idce, though they 

 abounded in the ancient seas of every age, from the Silurian to the 

 Tertiary, and twenty-five or thirty families are known only by their 

 fossil remains. In these families more than 2,000 species have been 

 described. 



The Xautihis pompiHus, it Is said, inhabits a depth of from 20 to 30 

 fathoms, where it creeps, like a snail, on the bed of the sea. The 

 chambers of the shell, not occupied by the animal, are said to contain 

 water. The animal is attached to the shell, by means of two lateral 

 adductor muscles, and by a continuous horny girdle around the mouth 

 of the body chamber. The septa arc pierced by a siphuncle, contain- 

 ing, for a short distance, a tubular prolongation of the mantel, Avhich, 

 in the young state, probably forms the main muscular attachment. 



Profi Owen says: "The line of atttachment of both the muscles 

 and the cincture progressively advances with the growth of the ani- 

 mals. A certain portion of the fundus of the shell thus becomes va- 

 cated, and the Naittilus commences the formation of a new plate,^ for 

 the support of the part of the body which has been withdrawn, from 

 the vacated shell. The formation of the plate proceeds from the cir- 

 cvimference to the center, and there meeting the conical process of the 

 mantel, which retains its primitive attachment, the calcification is con- 

 tinued backward for a short distance, around the process, which now 

 forms the commencement of the membranous siphon, and ac(|uires the 

 partial protection of the calcareous tube. An air-tight chamber is thus 



