1883.] 323 [Hyatt. 



the gradations of the forms and sutures are unmistakable and 

 connect them genetically with the genus Parodiceras. The si- 

 phonal saddles are evidently in this and other families purely 

 representative characteristics arising from the tendency to divide 

 the primitive ventral lobes. Within the families, however, it is 

 evident that the siphonal saddles are inheritable after they have 

 been introduced, and become fixed in the organization. In each 

 of the three sub-families, the siphonal saddles are in accordance 

 with this law first independently generated, and then become fixed. 



The essentially representative character of the division of the 

 lobes and saddles when first introduced in each series is especially 

 well exhibited in the Dimorphocerae. In this tribe the ordinary 

 outlines of the sutures of this family are modified by the introduc- 

 tion of marginal saddles, which sub-divide the lobes, as among the 

 more recent Ammonitinae. After careful investigation, we can find 

 no evidence for the supposition that the recent Ammonitinae ac- 

 quired their similar modes of dividing the lobes by direct inheri- 

 tance from such highly involute Carboniferous species. On the 

 contrary there is strong evidence that the Triassic Ammonitinae 

 sprang from discoidal shells with forms of whorl more closely 

 resembling the primary radical Anarcestes. 



If we analyze the forms of Triassic species, as they have been 

 published by Mojsisovics in his classic work Med. Trias., Prov. we 

 are at once struck by the prevalence in every series of a certain 

 proportion of discoidal forms, and by the fact that these are 

 repeated in the young of the more involute forms of the same 

 series, as is not infrequently noted by Mojsisovics himself in his 

 descriptions. Some genera like the Sphingites among Arcestidae 

 and Tirolites, and Xenodiscus may have all, or nearly all the 

 species discoidal, but as a rule the variation from these forms to 

 much more involute, or completely involute forms takes place in 

 the same genus, and is useful only in distinguishing the species. 

 The discoidal or less involute forms are always the simplest 

 in the structure of the sutures, as well as larval in their own 

 series ; witness again Sphingites and Xenodiscus, also such single 

 species as are found in the group of Trach. furcosa, and Mono- 

 phyllites. 



Arcestes, Dya;s, and Trias, so far as we know, is the only group 

 as a whole, which possesses in the adults the depressed larval 



