INVERTEBRATE PALEONTOLOGY. 411 



The original typical species of this genus only show the shell folded hack 

 upon itself once. But none of the specimens are entire, the larger folded 

 portion being broken so that it is not possible, without seeing more complete 

 examples, to be sure that the shell may not have made another curve back 

 upon itself, even in these species, as in Hamites. Indeed, this reduplicate 

 character has been seen unmistakably present in one of the Indian species, and 

 some of the European, as well as in one of those described by Mr. Gabb from 

 the Cretaceous rocks of California. Upon this character, Mr. Gabb has pro- 

 posed to found a genus Diptychoceras ; and if we could be sure that any one 

 of the type-species of Ptychoceras only folds back upon itself once, in the 

 mature shell, there would seem to be sufficient reasons for the division of the 

 genus thus proposed. Until this question can be settled, however, by more 

 nearly complete specimens, the propriety of such a division must remain 

 doubtful, particularly as it seems quite as probable, judging from analogy, that 

 the reduplication of the shell may take place in adult examples of the typical 

 species of Ptychoceras, as in Hamites* 



Mr. Conrad has also proposed a genus Solenoceras for the reception of 

 Ptycl/oceras annulifer (= Hamites annulifer, Morton), a small species that 

 not only had its limbs in contact, but the smaller limb received into a deep 

 furrow along the inner side of the larger. It shows no positive evidence, of 

 more than one folding upon itself; but then the specimen is broken off at both 

 ends, so that we can hardly be sure as to the exact form of the mature 

 unbroken shell. At the larger of the broken ends, there is, in the type- 

 specimen, some slight appearance of the commencement of another bend, 

 outward or away from the inner limb. Unfortunately, however, the specimen 

 is hardly in a condition to be altogether satisfactory on this point. It looks 

 almost as much like the remains of a kind of margined lip, as if the shell had 

 formed there one of those premature thickenings, or constrictions, of the lip, 

 that we have reason to believe were sometimes formed in allied genera during 

 the growth of the shell, to be left behind, or re-absorbed as the shell increased 

 in size. If, however, it did make a bend there in the direction supposed, 

 Solenoceras would certainly be a good genus. If not, I should think the 

 more deeply-embracing characters of the larger limb could hardly alone form 

 a sufficient generic distinction, as this is a character in which the known 



* Of course, in young or half-grown shells, the second folding hack upon itself never occurred, 

 this character only being developed in the adult. 



