DEVONIAN PERIOD 43 



(Palaopalcemori), forerunners, probably, of great develop- 

 ments in crustacean life. 



Devonian ostracods have left no record of much importance. OSTRACODS 

 The little creatures, with notches in their shells, through 

 which they could thrust their antennae, were certainly 

 prosperous (Entomis, etc.). Less fortunate were those 

 that could only protrude the appendages by gaping the shell- 

 valves. 



A few isopods are first met with in this Period. This not ISOPODS 

 very respectable group of crustaceans comprises sea-slaters, 

 louse-crabs, gribbles, woodlice, and pill-bugs. In Devonian 

 times, however, these isopod potentialities were concentrated 

 within the narrow bounds of a few indefinite creatures 

 (Prczarcturus, Amphipeltis). 



Molluscan life was becoming more prominent than of old. 

 Its numerous species for the most part greatly differed from 

 modern molluscs, but there were some clear signs of modifica- 

 tions in the direction of existing forms. 



Among cephalopods there is no evidence, it is true, of 

 cuttle-fishes and squids which now represent so largely 

 this leading class of molluscs ; but great movements were hi 

 progress. 



The Nautiloids apparently were not so flourishing as in 

 former times. Old-fashioned forms with more or less straight 

 shells had become impoverished by the loss of some genera ; 

 and seem now to have been surpassed in numbers by those in 

 coiled shells of the nautilus type. 



The most notable cephalopods of the time were certain AMMONOIDS 

 molluscs, closely allied to the nautilus-like creatures, and 

 known as Ammonoids. Their shells, as a rule, somewhat 

 resembled a ram's horn in shape ; and the Order for that 

 reason has been named after the Egyptian god Ammon, 

 generally represented as a man with a ram's head. 



Some of the ammonoids of this Period had shells much of 

 the shape of those of the straight-shelled nautiloids (Bactrites). 

 The ram's horn name, therefore, is not strictly applicable 

 in their case ; but as in certain internal features of the shell 

 they differed from nautiloids, and resembled typical am- 

 monoids, they are classed with the latter. They were probably 



