1883.] 303 [Hyatt. 



ceratites, like C. carolinum sp. Mojsis. Med. Tr. Pro v. pi. 83. N". el- 

 egans, Meek, and N". laevigatus are, however, transitional between 

 this genus and Cymatoceras, and connect them closely. Thus we 

 are obliged to regard the last as an offshoot of the genus Nauti- 

 lus or of Cenoceras. 



MACROCHOA^ITES. 



(Transitiones.) 



This group necessarily includes forms which belong to both 

 Nautiloidea and Ammonoidea so far as their embryonic stages 

 are concerned. Thus Bactrites, so far as we know, is a true 

 Nautiloid shedding the protoconch and presenting the cicatrix 

 on the apex of the conch or true shell, as figured by Barrande. 

 Syst. Sil. pi. 490, from drawings made by the author. The ven- 

 tral sutures are interrupted by the funnels as in Ammonoids, and 

 the sutures are otherwise similar to those of Mimoceras and An- 

 arcestes in some forms. These resemblances are precisely similar 

 to those habitually occuring only between closely allied, parallel 

 series in the two orders, which have been traced to a common 

 radical. The inference is unavoidable, therefore, that we are 

 here dealing with forms which have all sprung at no very remote 

 period from an ancestor with a slender whorl, and striae of 

 growth and sutures like those of the simpler and purely orthocer- 

 atitic forms of Bactrites. Having found it impracticable to intro- 

 duce the curious aberrant forms of the Clymeninae in any other 

 way, we have placed them in the form of a note. 1 



Bactritidae. 



Bactrites, Sandb. Leonh. et. Bron. Jahrb. 1841, p. 240, is really 

 a synonym for Trematodiscus, Eichw., Bull. Soc. Imp. Nat. de 

 Moscow, p. 200, but it seems inexpedient to try now to restore 

 the original applications of these names. One cannot compare 

 Bactrites and Mimoceras compressum without being struck by the 

 close affinities of the two forms, and feeling disposed to join 

 Quenstedt in his, at that time, daring thought, that these two 

 forms were closely allied, and bore to each other the same rela- 

 tion as the straight Baculites and the associated coiled Ainmoni- 



1 See "Note" p. 312. 



