592 MOLLUSCA SUB-KINGDOM VI 
period, but there are numerous localised signs of retrogression, due perhaps to un- 
favourable surroundings. Indications of this kind occur sporadically throughout the 
Jura and become general in the Cretaceous, leading us to infer a widespread unfayour- 
able change in their physical surroundings, similar but more extensive than that which 
affected European forms during the Inferior Oolite. To the latter influence should 
probably be ascribed the uncoiling observed in the Spzroceratidae. With the close of 
the Cretaceous all species disappear for ever from the pages of geological history. 
TABLE SHOWING RANGE AND PRESUMABLE KINSHIP OF 
AMMONOID SUB-ORDERS. 
ee em 




= = 
Cretaceous = aS 
= = 
SAA S 
— — a, 4. 
+2 | n> 
2 a 
Jurassic 2 3 
| | 
as 2! a SL 
Triassic | oes 
= | = 
| ears 
a ! ao mies 
2 
3 
Permian = 
— 
Carboniferous 
or eS | = 
Sy cs oo S = 
=e mal a) aa = 
5 ay = = 5 
A | Sy | = 3 = 
Deyonian || S Sy 3 | =) So 
2 Oo ¢ of So = 
Se) = ro nm re 
| a o =| iS) = 
ae ss SI os ed | 
Oo = <3) ab) ow 
si 
Micro-:campyli 

[Professor Hyatt’s revision of the Nautiloids and Ammonoids for the present work 
terminates at this point. The classification and diagnoses are condensed from an exhaustive 
Monograph on fossil Cephalopods, at present still in MS., which embodies the results of his 
life-study. Reference should be made to this work, when published, for a more complete 
account of families and genera than it has been possible to give in the preceding pages.— 
TRANS. ] 
Sub-Class 2. DIBRANCHIATA. 
Cephalopods with only two arborescent gills in the mantle-cavity ; provided round 
the mouth with eight or ten arms bearing suckers or hooks, two of them (when ten in 
all are present) being often developed into long tentacles. Funnel closed ; ink-sae 
usually present. Shell internal, or if external, is not chambered ; in many forms 
entirely wanting. 
