CARBONIFEROUS FOSSILS OF IRELAND. 23 



Genus Nautilus. 



Gen. Ch. — Shell convolute, discoid; septa simple at tlieir edges, concave outwardly; siphuncle discon- 

 tinuous, usually central ; aperture large, sinuate at the dorsal margin. 



There are several subgcneric types in even the genus as here restricted, but of these we shall take for the 

 present no notice; most of the Palseozoic species have the spire exposed in all its turns by a large umbilicus, 

 while there arc others in which the whorls are completely concealed by the last volution, such, for instance, as 

 the A^. dorsalis. 



Nautilus cyclostomus. Phil. 



Nautilus cyclostomus. Phil. Geol. York. 



Sp.Ch. — Apex spirally incurved, last chamber disjoined; nearly straight conic; mouth and section of the 

 whorls circular ; septa with a slight retral wave on the back ; siphuncle nearly dorsal ; surface smooth, or 

 marked with faint transverse sigmoidal stn£e. 



This cmious fossil approaches the genus Hortolus in structure, having the last whorl disjoined from the 

 rest, and produced nearly in a straight line ; it is rather a rare species. 



Nautilus dorsalis. Phil. 



Nautihis dorsalis (var. y.) Phil. Geol. York. 



Sp. Ch. — Shell rapidly enlarging ; last chamber very large ; mouth nearly circulai', receiving the preceding 

 whorl at its ventral margin ; umbilicus very small, rounded ; septa about twelve in each turn ; siphuncle 

 dorsal ; surface rugose. 



There are three varieties of this magnificent shell mentioned by Professor Phillips, but as yet I have only 

 seen the one above mentioned ; the surface is perfectly smooth in the most ordinary state of preservation, or 

 when a portion of the outer shell is deficient. A specimen in the Collection of the Royal Dublin Society has 

 a portion of the shell perfect, shewing it to be marked with small, rough, irregular, transverse wrinkles. The 

 back is rounded, and the whole outline much resembles that of the recent Nautilus pompilius. Greatest dia- 

 meter five and a half inches, thickness of whorls three and a half inches. (Small specimen). 



Nautilus gootolobus. Phil. 



Nautilus goniolobus. Phil. Geol. York. 



Sp. Ch. — Subglobose, smooth, whorls concealed ; umbilicus very small, rounded; septa with a deep retral 

 wave on the back, and an angular lobe on each side. 



I have only seen a very imperfect specimen of this species. It is evidently not a true Nautilus, but I 

 hesitate to place it elsewhere, not having seen perfect specimens. 



Bellerophon. Montfort. 



Gen. Ch. — Globose, involute, monothalamous ; last whorl embracing the others; a deep sinus in the mid- 

 dle of the outer lip, from which a band passes round the whorl, forming a kind of keel ; surface transversely 

 striated. 



The present genus differs remarkably from the other testaceous Cephalopoda, with the exception of Aryo- 

 naufa, in having no chambers, and in the deep sinus which exists in the Up ; from Argonauta it differs in the 

 greater thickness of the shell, and more involute form, and the distinctly marked inner lip found in most of the 

 species ; these characters are also quite incompatible with the station assigned to it among the Heteropoda, by 

 modern writers ; I am not aware on what grounds it has been associated with the Gasteropoda by Professor 

 Fleming. The genus occurs in the upper and lower Palaeozoic rocks. 



