446 Frof. H. A. Nicholson — On the Genus Ortonia. 



IV. — On Ortonia, a New Genus of Fossil Tubicolar Annelides, 

 WITH Notes on the Genus Tentaculites. 



By H. Alleyne Nicholson, M.D., D.Sc, M.A., F.R.S.E., 



Professor of Natural History and Botany in Uuiversity College, Toronto. 



HAVING recently liad the opportunity of carefully investigating 

 tlie genus Tentaculites, I was led to the conclusion that 

 several fossils of diverse zoological afSnities had been included 

 under this head, and that the prevalent dilferences of opinion as to 

 the systematic position of this genus might be thus readily explained 

 (American Journ. of Science and Arts, vol. iii., no. 15, 1872). 

 Originally founded by Schlotheim in 1820 (Petrefact. i., p. 377), 

 Tentaculites, as its name implies, was believed to comprise fossils 

 which were nothing more than the slender terminations of the 

 jointed arms of Crinoids. Modern palaeontologists have been 

 divided in opinion as to whether Tentaculites was truly referable to 

 the Tubicolar Annelides or the oceanic group of the Pteropoda — 

 most recent authorities placing the genus in the latter class. In 

 point of fact the difficulty has really arisen from the circumstance 

 that two dissimilar sets of fossils have actually been included under 

 Tentaculites ; some of these being genuine Pteropods, whilst others 

 are equally genuine Tubicolar Annelides. As all the typical species 

 belong to the first group, it follows that the genus Tentaculites 

 remains a Pteropodous one ; whilst the Annelidan forms must be 

 referred to new genera. 



The restricted genus Tentaculites, then, may be defined as in- 

 cluding small shells which have the form of straight conical tubes, 

 tapering towards one extremity to a pointed closed apex, and ex- 

 panding towards the other to a rounded aperture. The shell is free, 

 and its walls are thin, and are surrounded by numerous thickened 

 rings or annulations, sometimes with intermediate strise, over the 

 whole or part of the length of the tube. 



The two points by which Tentaculites may be distinguished from 

 all homomorphous forms are — firstly, that the shell is straight, or if 

 bent, regularly bent or curved; and secondly, that the shell is free. 

 It is quite clear that if, as there is every reason to believe, the 

 genuine forms of Tentaculites are truly referable to the Pteropoda, 

 every example must conform to these two characters. No member 

 of the genus can possibly have been attached to any foreign body ; 

 and none can have been irregularly twisted or bent. The shell in 

 all genera of Pteropods is free, and in all is either straight or is 

 regularly curved. 



On the other hand, whenever we meet with apparent examples of 

 Tentaculites which are attached parasitically to shells or other foreign 

 bodies, or which have the shell irregularly contorted, we may be 

 quite sure that we cannot be dealing with any Pteropod. We are 

 dealing now with Tubicolar Annelides, and it is a matter of astonish- 

 ment how close a superficial resemblance is presented between some 

 of these and specimens of Tentaculites proper. 



In accordance with the principles here laid down, I have already 



