368 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D Ser. 



9 



Perhaps no other genus of ammonites has been so vari- 

 ously treated as Meekoceras, for the reason that it is very 

 variable, and also because in the first description no type 

 was expressly given, and three species differing in essen- 

 tial respects were simultaneously described, Meekoceras 

 aplanatuni White, M. 7Hushbacha7iutn ^^hiio., M. gracilitatts 

 White, in the order given. But in the diagnosis of the 

 genus Hyatt first mentions M. gracilitatis. Thus, accord- 

 ing to usage, either M. aplanatum or M. gracilitatis might 

 be taken as the type. Of these only the latter agrees with 

 the generic diagnosis, since it has the fourth lobe as an 

 auxiliary series of denticulations, and does not have a dis- 

 tinct saddle between this lobe and the umbilical suture. 



Besides the species mentioned, Hyatt also included in 

 the list of species of this genus previously described species 

 that have since been assigned to Balatonites, Hitngarites, 

 Xenaspis^ and Celtites, which have clearly no relationship 

 with the typical members of the genus. 



The writer regards all species that agree with any one of 

 the three species, M. aplanattmi, M. muskbackanum, and 

 M. gracilitatis, as belonging to Meekoceras in the broader 

 sense. 



Mojsisovics^ was the next to treat of this genus, in which 

 he included a number of species now assigned to Profty- 

 chites and Beyrichites, thus giving Meekoceras din unwar- 

 ranted extension beyond the limits assigned by Hyatt. In 

 a later paper Mojsisovics^ still further extended the genus 

 in one direction to take in forms now assigned to Heden- 

 strcetnia, but limited on the other side to involute forms. 

 All evolute, open-coiled forms were assigned along with 

 M. aplanatuni to Xetio discus Wa.?igen, although Mojsisovics 

 confessed that this was purely because Xenodiscus seemed 

 to be the ancestor of a different stock. None of the forms 

 assigned by Mojsisovics \q Xenodiscus agree with Waagen's 

 genus, and they are no longer classed under it. 



Meekoceras was next treated by Waagen^, who regarded 



' See Bibliog. 21— p. 213. 

 2See Bibliog. 20— p. 79. 

 3 See Bibliog. 38— p. 236. 



