i899- Foord.- — Carboniferous Brachiopoda and Moltusca. 83 



" umbilicus/' caused by the spiral growth of the shell coupled 

 with the very slight overlapping of the lateral portions of the 

 whorls. This construction is well exemplified in a recent 

 shell, well known to conchologists — the Solarium of the 

 eastern seas. 



In Pleurotomaria we have a survivor from the earliest 

 marine fauna long supposed to be extinct till some years ago 

 it was dredged alive off Tobago and in the Japanese seas. 1 

 This shell has an elegant, conical spire, and there is always a 

 deep notch, or slit in the margin of the aperture corresponding 

 with a band (the filling up of the notch) which encircles each 

 whorl, and is a useful distinguishing mark in the fossil shells. 

 Naticopsis, a shell, as the name implies, something like the 

 Natica of the present oceans, is often met with in the Car- 

 boniferous Iyimestone. The spire is proportionately very small, 

 and the body-whorl very large and of a rotund form. The 

 shell is generally thick, and is often extracted from the rock 

 intact and showing the most delicate surface markings, as if 

 the shell had just been taken out of the sea. In Loxonema 

 the shell has many whorls, very gradually increasing in size, 

 thus giving it a steeple-like aspect. Most of the species have 

 distinct S-shaped lines of growth crossing the whorls 

 obliquely. These suggested the generic name. 



The fossil Cephalopod shells that we are concerned with 

 may be roughly divided into the u?icoiled, including straight 

 and curved ones, and the coiled. Among the former Ortho- 

 ceras is characteristic ; its type of structure is essentially that 

 of the Nautilus shell, which, if it could be uncoiled and 

 straightened out would match very well the more bulky 

 conical forms of Orthoeeras. Hence, shells of the class under 

 discussion are called natitiloid, and they belong to a great 

 group or sub-order called the Nautiloidea. The salient feature 

 in this group is the simplicity of the " suture-lines," that is, 

 the edges of the septa where they abut against the walls of the 

 shell. These lines, as seen on a cast from which the shell has 

 been removed, maybe more or less distinctly curved, but they 

 never form zigzags, or foliations, as in the group of the 

 Ammonoidca, or Ammonite-like shells, presently to be re- 

 ferred to. 



1 A fine specimen of this shell from Japan was exhibited about twelve 

 years ago at the British Museum. 



