VESTINAUTILI'S CRASSIMARGINATUS. 79 



pi-imarily of a series of ridges or keels extending from the margin of the 

 umbilicus to that of the ventral area. These ridges, which are beautifully 

 crenulated by the lines of growth which cross them transversely almost at a right 

 angle, vary in strength and distance apart. There are nine of them ; those of the 

 umbilical margin, of which there are three, being the strongest, the other six 

 becoming finer as they approach the ventral border. The widest space between 

 the ridges is about the middle of the lateral ai-ea ; on either side of this zone they 

 are closer together. This description is drawn up from the test on the body- 

 chamber of an adult (?) specimen ; the ridges are more regularly spaced on the 

 test of the adolescent stage. The ridge just inside the umbilical border is quite 

 strong in the adult shell, in the adolescent it is exceedingly faint. There are 

 four ridges on the peripheral area, viz. two on each side, near its border, the inner 

 ones being exceedingly fine thread-like lines. These ridges are quite distinct in 

 the young shell, but in the adult the inner ones disappear upon the body-chamber. 



A marked feature in the ornamentation is the smoothness of tlie zone en- 

 circling the umbilical cavity below its margin and extending from thence to the 

 suture or line of junction with the preceding whorl. I have called this zone 

 smooth, though there is a faint ridge or flattened band encircling it in the 

 adolescent shell, but this compared with the strong, crenulated lateral ridges may 

 well be neglected. 



Affinities. — I am not at all confident that the generic position of tlie present 

 species is correctly determined ; it certainly approaches nearest to Vedinaatilus in 

 the configui'ation of the whorls, and especially in the form and the ridged 

 character of the periphery. The wide lateral areas, so strongly ridged, with the 

 smooth, unornamented umbilical declivities are, however, features without a 

 parallel in that genus. Naturally, the question as to what species it is most nearly 

 related to is still more difficult to decide, as none resembles it sufficiently to 

 warrant any strict comparison being made. The species must remain for the 

 present isolated. 



Remarks. — This very beautiful species was found at Lisbane, in the county of 

 Limerick, associated with the species I have called Triholoceras formosuvi. Only 

 two specimens were collected ; both of them are in the Museum of Science and 

 Art, Dublin (Collection of the Geological Survey of Irelmid). 



Locality. — Lisbane, county of Limerick. 



Vestinautilus cHAssiM.ui(iiN.\Tus, sp. uov. Plate XXII, figs. 5 a — c. 



Descrijition. — Shell discoid, with slowly increasing whorls, all exposed in ;i 

 deep umbilical cavity having a large central vacuity. The whorls are digonous 



