[NVBETEBEATE PALEONTOLOGY. 507 



their branchiae, as well as in the regularly-coiled, Ptanorbis-Yike form of the 

 tube of Spirorbis, they seem to be sufficiently distinct to be separated gener- 

 ically.* 



The shells of these annelids were long supposed to be those of true 

 mollusks ; but as soon as naturalists examined the animal inhabiting them, it 

 was found to belong to the Arliculata. Where we only know the shelly tubes, 

 however, as is of course always the case with extinct species, it is very diffi- 

 cult to distinguish species of this genus, not merely from the allied genera, 

 but even from Vermetus, a true mollusk. Consequently, much uncertainty 

 exists in the classification of the fossil species, and for the same reason the 

 geological range of the genus is not certainly known. Some authors refer to 

 it Devonian and even Upper Silurian species; but it is quite probable that if 

 we had the means of ascertaining the nature of the animal once inhabiting 

 these shelly tubes from the older rocks, they would be found to differ gener- 

 ically from the more modern and existing Serpulas. The number of species 

 supposed to belong to this genus is found to increase as we ascend through 

 the Carboniferous and later formations, and the genus appears to attain its 

 maximum development at the present time. The recent species are num- 

 erous, and attach themselves to stones, shells, pieces of wood, the bottoms of 

 ships, &c, and are widely distributed. * 



Serpnla! tcnnicarinata, M. & H. 



Plate 6, fig. 1. 



ScrpuJa Icnuicarinata, Meek and Haydeu (1857), Proceed. Acad. Nat. Sci. Philad.,IX, 134; and (1860) ib., 

 419. — Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 26. 



Tubes growing in groups, or rarely single, nearly cylindrical, increasing 

 very gradually in size, irregularly curved, but apparently never spirally coiled, 

 attached by the under side throughout most of the entire length ; upper side 

 having a distinct, rather sharply elevated, flexuous, longitudinal carina ; sur- 

 face smooth. 



Length unknown; average transverse diameter, 0.14 inch. 



* Dr. Stoliczka has proposed (Paleeont. Indica, II, 237, 1868), a genus Tnbulostuim for the recep- 

 tion of a group of planorbicnlar and spirally-coiled shells which have the aperture somewhat produced 

 and contracted. He regards them as the shells of mollusks, which their structure, as seen in sections, 

 seems to warrant, though tbeir general appearance is much like that of the articulate genus Spirorbis. 

 However this may be, I think the name Titbulostium superfluous, as I have the impression that Bronn 

 had long since named the same group Sj>irul(ra, and Defiance, I think, had also called it Botularia — both 

 being founded upon Scrpida spiruJaa, Lamarck. 



