INTEODUCTION. 
Up to the time that Professor Hyatt’s preliminary memoir upon the 
Fossil Cephalopoda appeared (1883) \ no serious attempt had been 
made to separate the Xautilidoe into distinct genera, the divisions 
accepted by palaeontologists being those of Quenstedt ” or some 
modification of his system. Although d'Orbigny M‘Coy and 
Meek and Worthen ® established a few groups — Cryptoceras^ Dimtes^ 
TemnocJieilus, Trematodiscus, Solenocheihis, &c. — these were gene- 
rally held to be merely of suhgeneric value, and by some authors, 
notably de Koninck ® and W. Waagen they were rejected alto- 
gether. 
Professor Blake ® has separated the Xautiloidea into four groups 
— Conici^ Injlati, Spirales, Irreyidares — of which Xautilus and Gyro^ 
ceras constitute Group III. {Spirales)^ Chymenia, Discites, Xothocems, 
and Atuna being treated as subgencra of Nautilus. 
Mojsisovics ° has divided the Triassic forms into groups, some of 
which arc equivalent to those of M‘Coy and of ^Icck and AVorthen, 
while others {Pleuronautilus and Chjdonautilus) are his own. He 
appears to have been the first to endeavour to trace out the deve- 
lopment in time of a series of these forms, particularly in the case 
of Pleuronautilus {infra, p. 135). 
^ Proceed. Boston Soc. Nat. Hist. 1883, vol. xxii. p. 253. 
^ Petrefactenkunde Deutscblands, Band i. 1840-49, pp. 52-60. 
^ Prodr. do Pal4ontologie Stratigraphique, 1849, vol. i. p. 114. 
^ Synop. Carboniferous Foss. Ireland, 1844. 
® Proc. Acad. Nat. Sci. Philadelphia, 1801, p. 147 {Trematodiscus). Ibid. 
1870, p. 47 {Solenochcilus). 
® Faune du Calcaire Carboniferc de la Belgique (Annales du Mus. Eoy. 
d’Hist. Nat. de Belgique, vol. ii.), pt. i. 1878, p. 85. 
Mem. Geol. Surv. India — Palseont. Indica — ser. xiii. vol. i. pt. i. May 
.1879, p. 43. 
® British Fossil Cephalopoda, pt. i. 1882, p. 47. 
® Die Cephalopoden der Mediterranen Triasprovinz (Abhandl. d. k.-k. geol. 
Reichsanst. Band x.), 1882, pp. 265-290, 
