194 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



knots forming a pair of marginal keels on the abdomen ; 

 shell with obscure knots or ribs, but with fine sigmoid lines 

 of growth ; septa divided into a large number of lateral lobes 

 and saddles, increasing in size up to the third, and decreas- 

 ing from that toward the umbilicus. The type of the genus 

 was Placenticeras placenta Dekay, of the Fort Pierre group. 

 Upper Cretaceous. Along with the type species Meek 

 included in this group also P. andoorense Stoliczka, P. 

 guadaloupcB Roemer, and P. orhignyanum Geinitz, all from 

 the Cretaceous. Meek did not attempt to place Placcntice- 

 ras in any of the so-called families of ammonites, but by 

 almost universal consent paleontologists have grouped it 

 with the Amaltheidse (Zittel, 1885, p. 452; 1895, p. 407; 

 Steinmann, 1890, p. 416), along with the so-called "Cera- 

 tites" of the Cretaceous, on account of a certain resem- 

 blance to the Jurassic forms with beaded abdominal keels. 

 This, however, was before the days when paleontologists 

 looked upon the development of ammonites as the key to 

 their systematic position, and taxonomy made little pretense 

 of being biogenetic. 



The first dissenting voice was raised by H. Douville, in a 

 paper "Classification des Ceratites de la Craie" (1890), 

 where the opinion was expressed that Placenticeras and 

 Sphenodiscus both developed out of the group of Hoplites- 

 Sonneratia. F. Bernard (1895, p. 676) has accepted this 

 view, placing the genus under the Stephanoceratidse. 

 Unpublished researches of the writer show, however, that 

 Sonneratia and Desmoceras are much more closely allied to 

 Stephanoceras than is Hoplites, from which group Placenti- 

 ceras originated. Hoplites is considered by the writer, not 

 as originating from the Stephanoceratidas, but as having a 

 common origin with that family in the ^goceratidas. 



The species from the Lower Cretaceous commonly 

 assigned to Placenticeras mostly belong to Oppelia, to 

 Sphenodiscus, and to other genera. Not every keeled, 

 discoidal ammonite can be placed here, for it is well known 

 that among the ammonites mere resemblance is not proof 

 of near kinship. Sarasin (1893) has shown that Ammonites 



