122 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



This serious drawback is parth' redeemed by tbe excellent litbographic illustra- 

 tions of tbe fossils described in the text.' 



The generic affinities of Acaiithonaiitiliis are clearly those which are indicated 

 in Professor Hyatt's association of it in the family Solenocheilida? ; ■ and if the 

 spines had been absent the two species representing the genus would have fallen 

 naturally into the gemis Asi/mptoceras. 



Localit)/. — Clane, county of Kildare. 



Genus Asymptocekas, Ri/rkhoU, 1852 (emend. Hi/atf, 1883, 1893). 



AsYJiPTOCERAS CHA5SILABRUM, sp. uov. Plate XXXI, figs. 1, 2. 



Descripiion. — Shell nautilus-like iu general form and aspect, the whoi'ls 

 increasing rapidly, subquadrate in section from an early stage of growth, and 

 more distinctly so in the adult ; the sides broad and flattened, the periphery slightly 

 depressed along the median line, its breadth considerably less than that of the 

 shell at the umbilical margin owing to the upward convergence of the sides. 

 Umbilicus deepening rapidly as the shell expands, the initial whorl leaving an oval 

 vacuity. The aperture has apparently a shallow hyponomic sinus on the periphery, 

 and in front the lip forms a prominent rounded rim, caused by its inward folding. 

 This rim does not extend beyond the walls of the umbilicus. 



The body-chamber occupies about one-half of the last volution. 



The septa are very wide apart. Where the breadth of the periphery is 20 mm. 

 they are 13 mm. apart, where it has increased to 50 mm. in breadth they are 

 24 mm. apart; and the measurements of the last chamber give for the periphery 

 and septa respectively 55 mm. and 20 mm. 



The ventral siphuncle, characteristic of the Soleuocheilidae, is seen in a large 

 specimen, from which some of the shell has been broken away, iu the shape of a 

 slender tube about 3'5 mm. in diameter, partly buried in the rock which fills the 

 chambers. The test is apparently smooth everywhere except upon tlie rim which 

 borders the front of the aperture, where there is a series of fine and regular lines 

 running parallel with its edge. 



The impression of the shell muscles is plainly indicated on one of the speci- 

 mens before me. It consists of a fine iueised line about 4 mm. above the basal 

 edge of the body-chamber, running nearly parallel thereto in the umbilicus and on 

 the sides of the shell, but becoming slightly deflected upon the periphery, where 



' The copy of the work to which I fortunately gained access is contained iu the admirably 

 quipped library of the Geological Society of London, Burlington House. 

 "- Loc. cit. 



