1858.] MUECHISON NOETHEEN HIGHLANDS, ETC. 377 



the Goniatite or Ammonite, but specialized for a certain end. I had 

 occasion to point out this in a paper on a peculiar Cephalopod, 

 Tretoceras*, which has, in addition to the ordinary siphuncle, a long 

 posterior lobe extending through seven or more of the septa. In 

 some of Hall's figures of Endoceras {op. cit. pi. 44, for instance, if 

 not in pi. 19), there are indications of a forked or triple termination 

 to the long siphuncular lobe, the cavity behind being filled up by 

 sohd deposits. Hence there is an appearance as of one or more Or- 

 ihocerata being contained in the large siphuncle. The fossils called 

 Hyolites, in the Silurian rocks of Sweden, are believed to be similar 

 internal casts of siphuncles. 



In Piloceras the continuity of the hinder part of the body with 

 the siphuncular lobe is complete, and only ordinary septa (not solid 

 masses) are secreted from its surface. There is no appearance of any 

 external shell to which these conical septa might have appertained 

 as a broad siphuncle. 



We have then, in this fossil, what I believe to be the simplest 

 form of Cephalopodous shell; and it is very suggestive that it 

 should occur at the base of the Silurian deposits (the lowest in 

 which Cephalopoda are known) both in Britain and Canada f? and 

 be accompanied in these beds by genera, like Endoceras and Came- 

 roceras, which connect it with the ordinary forms of Orthoceratite. 



PiLOCEEAS ? 



There are not specimens enough to characterize a small species 

 which occurs of this genus, an inch or two long, and of a cylindrical 

 tapering form. The septa are of the same shape as those above 

 described. 



Oncoceeas ? PI. XIII. fig. 27. 



The specimens are so imperfect that we cannot tell if the mouth 

 be really much contracted or not, or of what form the apex is. The 

 latter is but slightly curved, and the shell increases gradually and 

 regularly towards the terminal chamber, which is somewhat longer 

 than broad, and appears to be inflated. The septa are concave and 

 oblique, rising highest against the dorsal (shorter) side, and so close 

 together that seven lie in the space of a quarter of an inch. The 

 siphuncle is rather large and external. 



The American species, 0. constrictum, is more curved and more 

 contracted at the mouth, and it has more distant and straighter 

 septa. Our fossil is referred very doubtfully to Oncoceras. 



aASTEEOPODA. 



Macltjeea Peackci, spec. nov. PL XIII. figs. 1-5. 



M. triuncialis, valde depressa, uinbilico latissimo ; anfractibus 5-6, infra planatis, 

 supra rotundatis subangulatis ; operculo longo spirali. 



This fine shell is distinguished immediately from the other known 

 Madureoe by its very flat shape, the discoid whorls scarcely over- 



* Quart. Journ. Geol. Soc. vol. xiv. p. 177. 



t In the Calciferous Sandi'ock, Billings (in letter), 1859. 



