46 KANSAS ACADEMY OF SCIENCE. 



The sutures are near each other, or slightly crowded in aspect. They have a 

 narrow abdominal saddle, deep, broad lateral lobes, comparatively narrow lateral 

 saddles near the umbilici, and a pair of shallow lateral lobes internally, on the 

 shoulders of the whorls. 



The shell is thin, and it is marked by fine lines of growth. The siphuncle is 

 probably situated near the abdomen, but was not clearly seen. 



A specimen sent me by Mr. Hay from Fort Riley is the most perfect specimen 

 of this remarkable species that I have yet seen. It has an almost entire living 

 chamber, about one-half of a volution in length, the sutures show well, and it is not 

 as much compressed as specimens from Texas. All the specimens are reported as 

 coming from carboniferous, as do all species of the genus so far found. 



The sutures may have a slight lobe on the hollow of the narrow abdomen, where 

 compression has affected them; where they are unaffected by compression, they are 

 absolutely straight or very faintly concave. In Mr. Hay's cast, the outer part of 

 the living chamber presents the abdomen as slightly convex, and leads one to think 

 that the slight hoUowness of the abdomen often present in younger whorls is due to 

 compression. In fact, the whorl is broken along a line parellel with and near to the 

 edge of the abdomen and is concave from compression on the right-hand (morpho- 

 logically left) side until near the end of the living chamber. Here, where the abdo- 

 men presents a very flat convex surface, both sides of the whorl are unbroken and 

 have the normal proportions. 



This is the largest and finest species of the involute shells of this group yet 

 found in the carboniferous. The principal differences between it and Nautilus 

 Rouilleri, the adult of which was described and figured by Trautschold [see Kal- 

 bruche von Miatschkowo, p. 28, pi. 3, fig. 7,] under the name of oxystonms, [the 

 name Rouilleri was given to this as the type in De Koninck's Calcaire Carbonif6re, 

 p. 124, in his description of Nautilus oxystomus, which last was afterwards taken by 

 the writer as the type of his genus Phacoceras, in "Genera of Fossil Cephalopods,"' 

 Proc. Bost. Soc. Nat. Hist., XXII, 1883, p. 292,] and the young by Marie Tzwetaev, 

 [see op. cit., p. .53, pi. 6, figs. 33 and 34,] consist in its size. The principal differ- 

 ence between the European and American is that the former retains throughout life 

 — that is to say, on all parts of its largest whorl, which is much larger than that of 

 the European species — the peculiar but flattened abdomen which is found only in 

 the young of Phacoceras Rouilleri, This character is of genetic importance, and, 

 together with the longitudinal ridges and form of the young in this species, and in 

 P. oxystotnum, show that these acute involute shells were derived by descent from 

 more discoidal shells, like those of the genus Discitoceras. This also serves the 

 purpose of explaining the occurrence in the carboniferous of their apparently 

 anachronic forms and structural characteristics. The aspect of the adults and the 

 sutures in this genus are like triassic species, such as Grypoceras (Nautilus) galea- 

 tus Mojsisovics, and at first they appear to have occurred before their proper geo- 

 logic period. When, however, their young are studied, it is plain that their shells 

 at early stages have the ordinary characteristics of normal members of the carbon- 

 iferous faunas, and that the peculiarities of later stages were evolved from purely 

 carboniferous forms. Their mimicry of triassic shells in later stages must there- 

 fore be regarded simply as good examples of parallel progressive complications 

 arising independently in different genetic series during different periods of time. 

 In Rouilleri the flattened aspect of the crest of the abdomen is retained much longer 

 in the course of the growth than in Phacoceras oxystomum. The American species, 

 with its truncated abdomen existing in the adult, is therefore the most immature 

 form of the group yet discovered, and although it is as yet impossible to come to 



