CRETACEOUS FOSSILS. . 143 



inches; greater diameter of aperture of same, 1.3 inch; lesser diameter, 1.15 inch. 

 It is very probable that these fragments do not all belong to the same individual. 

 The specimen figured in Vol. 1, Pal. Cal., seems to be more nearly in proportion 

 to the spiral herein described. That fragment shows the recurve, and on one or 

 two of the ribs of the thinnest part, can be seen tubercles corresponding in char 

 acter to those described above. 



DIPTYCHOCERAS, N. Gen. 



Three straight limbs iii contact, partly enveloping. 



This genus bears the same relation to Ptychoceras that Hamites does to Hamu- 

 lina. It is a Ptychoceras in every respect, except that it has an additional limb 

 which incurves, enveloping both the preceding to a slight degree only. 



Stoliczka and d'Orbigny both notice the occurrence of species of Ptychoceras 

 with more than two limbs, and the former author amends the definition of the 

 genus so as to cover the case. His description is as follows : " Elongated, lanceo 

 late, straight, once, twice (or many times?) reflected; the siphuncle dorsal, the 

 sutures divided into six lobes and six saddles, all of which (excepting the ventral 

 lobe?) are bipartite." With all deference to so high an authority as Dr. Stoliczka, 

 I am not prepared to accept this definition of Ptychoceras, unless, at the same time, 

 we modify the characters of several other genera of the family. If the number 

 of straight limbs is not of generic value, then why not include Baculites also ? In 

 the present state of our knowledge, it seems that there is a well-defined group of 

 species characterized by two straight, parallel limbs, the larger, or newer of which 

 never develops beyond a certain point on the length of the smaller ; while another 

 group, including P. Dupinianum, d'Orb., P. Forbesianum, Stol., and the following 

 species, has this larger limb continued and again reflected. Should it be ascer 

 tained that there is a gradation between these two, or that other species have more 

 than two reflections, then there will be good grounds for doubting the validity of 

 my genus ; until then, I believe we have sufficient reasons for maintaining it. 



Solenoceras, Conrad, founded on a single specimen of Hamites annulifer, Morton, 

 is a shell that had at least two straight limbs, one partially enveloping the other; 

 the greater of these has at its large end, a little offset or deflection, apparently 

 indicating an abrupt bending backwards, or outwards, of the shell, and not an in 

 curving, as in the present form. "Whether this was the case, what was the entire 

 form of Solenoceras, or whether it can be at all separated from Ptychoceras, are as yet 

 unknown. All the information we possess on the subject is obtained from a unique 

 specimen, which shows no peculiar generic characters, and which was, in plain 

 truth, separated from Ptychoceras only on suspicion. 



