Genera of Fossil Cephalopoda. 137 



as in other allied forms, but half-closed by the bending back of 

 the shell upon itself. Looking at the mouth of this shell, one 

 might think that for half its extent it had been closed by a sep- 

 tum, the direction of which is symmetrical and inverse to that 

 taken by the septum of the body chamber. On first examining 

 these specimens, 1 was inclined to think that half the aperture 

 was clo-ed by a loosened septum ; but farther observalions in 

 eight or ten individuals shewed that it could not be an accidental 

 condition, and lastly I found a specimen in which the whole cir- 

 cumference of the mouth could be traced with certainty. This 

 semiclosure of the orifice of the shell in Gyroceras, appears, there- 

 fore, to be analogous to the contraction of the mouth in Pragmo- 

 ceras, Gomphoceras and Lituites, above mentioned ; but is pecu- 

 liar in this, that it is not the lateral margins that are bent towards 

 each other, as in these genera, but only the inner (under) margin 

 is pressed back, {Journal of the Geological Society^ Vol. 10, jo. 23, 

 of Miscellanies,.^ 



8. Trochoceras. — Figs. 8 and 9 represent two forms of this 

 genus, the tube is coiled with a double curve like that of a snail, 

 and the whorls are either in contact as in Fig 8, or separate as in 

 Fig. 9. 



9. Ascoceras. — In this genus that portion of the shell which con- 

 tains the air chambeis, appears to be turned upwards, and recess- 

 ed into one side of the body chambers. A small portion of it 

 only remains at the bottom as seen at the base of Fig. 12. The 

 siphon in specimens denuded of the shell can also be seen at the 

 base. This genus has been recently discovered in the lower 

 Silurian Rocks of this country, by Mr. Richardson, of the Geo- 

 logical Survey of Canada. 



Fig. 10, is a longitudinal section of an Orthoceras with a large 

 marginal siphuncle. The spaces marked with the letter O, are 

 the air chambers. AAA the large chamber of habitation which 

 according to the views of M. Barrande, the distinguished palaeon- 

 tologist of Bohemia, extended into the siphuncle. In the lower 

 part of the siphuncle is a space marked B of a lighter colour. 

 M. Barrande is of opinion that certain species of this genus 

 secreted a calcareous matter in the siphon which at length par- 

 tially filled the tube. The letters B B indicate the position of this 

 deposit in the specimen figured which is the Orthoceras communis 

 of Europe. Professor Hall considers the Orthoceratites of the 

 Trenton limestone, with these large marginal siphons to be gene- 



