335 



DiPLOMOCERAS NOTABILE. (N. Sp.) 



Plate 44, figs. 4, 4 a and 4 h. 



Shell very large when perfect, subcylindrical and transversely ribbed, 

 ribs simple, leaving no impress upon the cast. The only specimen known 

 to the writer is a nearly straight piece of the prolonged portion, from 

 Hornby Island, about ten inches and three-quarters long, and septate for 

 by far the greater portion of its length. It is slightly compressed at the 

 sides, broadly oval and not far from circular in transverse section. Near 

 the smaller end it measures forty-seven millimetres in its diameter from 

 the siphonal to the antisiphonal side, and thirty-eight mm. in its lateral 

 diameter. Near the larger end the corresponding measurements are 

 fifty -five mm. by forty-six. 



The ribs are numerous, closely and regularly disposed,, nearly trans- 

 verse, but slightly oblique, rounded, and about as wide as the shallowly 

 concave 'grooves between them. On the middle of each side there are 

 about eight and a half ribs to the inch near the smaller end, and seven at 

 the larger. In addition to the ribs there are two widely distant, narrow, 

 transverse constrictions, running parallel with them. 



The sutural lines, although well preserved, and exposed over a con- 

 siderable portion of the surface of one side of the specimen, are so com- 

 plicated and crowded that it is almost impossible to follow any one of 

 them quite continuously from the siphonal to the antisiphonal side. A 

 careful study, however, show that the septation of this specimen is essen- 

 tially similar to that of Hamites cylindraceus, as figured by d'Orbigny on 

 Plate 136, figure 4, of the Atlas to the first volume of the "Terrains Cretaces," 

 which is the type of Hyatts recently proposed genus Diplotnoceras. In 

 both there are six lobes, viz., two large laterals (the " lateral superieur " 

 and the " lateral inferieur " of d'Orbigny) on each side ; one siphonal lobe 

 and one antisiphonal. In both, also, the two lateral lobes, on each side, 

 are very nearly equal in size. But the lobes and saddles of the Hornby 

 Island specimen are still more numerously incised than are those of the 

 French fossil, and this may be easily seen by comparing the siphonal 

 saddle, the largest of the three accessory saddles between the first and 

 second laterals, and the antisiphonal lobe, of both. The figure of the 

 siphonal saddle of H. cylindraceus in the Atlas to the Terrains Cretaces, 

 for example, represents it as entire at the summit and only twice incised 

 on each side, whereas in the Hornby Island specimen, the same saddle is 

 twice incised at the summit and four times on each side. 



North-west side of Hornby Island, the specimen described and figured, 

 which has most of the test preserved on one side, but very little on the 



