128 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Tube calcareous, procumbent, variously curved or spirally coiled, growing singly 

 or in groups, attached to marine bodies; capable of receiving the entire animal. 

 Aperture at the larger extremity simple and rounded. 



Animal without feet; mouth not provided with tentacles. Branchiae large, pec- 

 tinated, flabellate, with bearded lacinaB, and a cylindrical filament at the base of 

 each, differing in length in each of the branchiae, the longer sustaining an orbicular 

 disk, or funnel-shaped operculum. 



This genus is closely related to Spirorbis, which some authors include as only a 

 section of the same group. As these two types, however, differ in their branchiae, 

 as well as in the regularly coiled, Planorbis-Ekc form of the tube of Spirorbis, they 

 are doubtless distinct genera. 



The shells of these Annelids were formerly supposed to be those of true mollusks, 

 but as soon as naturalists examined the animal inhabiting them, it was found to 

 belong to the Articulata. Where we only know the shelly tubes, however, as is of 

 course always the case with the extinct species, it is very difficult to distinguish 

 species of this genus, not merely from the allied genera, but even from Vermehis, a 

 true mollusk. Consequently much confusion exists in the classification of the fossil 

 species, and for the same reason the geological range of the genus is not well deter- 

 mined. Some authors refer to it Devonian, and even Upper Silurian species ; but 

 it is quite probable that if we had any means of ascertaining the nature of the 

 animals once inhabiting these shells from the older rocks, they would be found to 

 all differ generically from the more modern and existing Serpulas. The number 

 of supposed Serpulas is found to increase as we ascend through the Carboniferous 

 and later deposits, and the genus appears to attain its maximum development at the 

 present time. The recent species are numerous, and attach themselves to stones, 

 shells, pieces of wood, the bottoms of ships, &c., and are widely distributed. 



Serpula (UNDT.) 



(PLATE V, Fig. 4.) 



Tubes growing in small gronps, irregularly curved, slender, increasing gradually in size ; having a distinct 

 carina along the middle above, and a more obscure angle along each upper outer side, so as to give a subquadrate 

 external form to the transverse section. Under side flattened, and inclined to spread out a little on the surface 

 to which it is attached. Aperture and transverse section of interior circular. Surface apparently smooth, or only 

 having very obscure marks of growth. 



Length, apparently, never more than about 2 inches ; greatest diameter, about 0.15 inch. 



This species resembles rather closely some of the Jurassic forms figured by Gold- 

 fuss and others ; but all our specimens being worn or weathered so as to obliterate, 

 to a great extent, the more delicate surface characters, we do not feel warranted 

 in identifying it with any foreign species, nor are we clearly satisfied that it is new. 



Locality and position. Southwest base of the Black Hills. Lower part of the 

 Turassic rocks of that region. (Type, No. 219.) 



