210 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



Gtathioceras ? sp. Plate XLVIII, figs. 3 a, b. 



This small fragment of a rather widely umbilicated shell, the inner whorls of 

 which are minutely and regularly striated transversely, is very difficult to determine 

 specifically. It bears some resemblance to G. stenolobum, J. Phillips, but the 

 umbilicus is proportionally larger, and it is a more inflated shell than the latter. 

 As I cannot recognise in it the young of any species known to me I have not 

 ventured to name it. 



Synopsis of Families, Genera, and Species contained in this Monograph, with 

 descriptions of the Families and, Genera ; the new species marked with an 

 asterisk. 



Sub-order— NAUTILOIDE A. 



Family — Oethoceeatid^:, M'Coij, 1844. 



Longicones and Brevicones, with cylindrical siphuncles and widely separated 

 septa. Body-chamber large ; aperture with an entire peristome. 



Genus Obthocebas (Breyn, 1732). — Shell straight or slightly curved; elongate- 

 conical ; circular or subelliptical in cross-section. Septa concave, usually horizontal, 

 sometimes slightly oblique. Siphuncle of variable dimensions ; cylindrical ; central 

 or excentric ; calcareous deposits sometimes present. Septal necks short or 

 reaching to the next septum. Body-chamber large ; aperture entire, scarcely ever 

 contracted ; sometimes with a constriction below it. Surface of test frequently 

 with transverse or longitudinal ornaments (annulations, ribs, striae, etc.). 



Cambrian to Trias. 



a. Longicones. 



I. Group ~Lmv\k. (Species with smooth test.) 



Sub-group Cylindriformes. (Species with cylindrical siphuncle.) 



1. 



Orthoeeras Leinsterense.* 



7. 



Orthoeeras cylindracemn 



2. 



variabile.* 



8. 



,, 



Nerviense. 



3. 



Colei.* 



9. 



„ 



amabile (?). 



4. 



,, Nolani.* 



10. 





calamus. 



5. 



Sancti-DoulagM.* 



11. 





Porteri.* 



6. 



„ acre.* 



12. 





venabulum.* 



13. Orthoeeras perapproximatum.* 





