EOCENE PERIOD 137 



Owing to defects in the geological record fish-life seems 



to have suddenly attained a decidedly modern aspect. 



' Writ in water " largely applies to fish genealogies. And 



indeed practically all life is met with in the Eocene after a 



break in its history. 



Among the invertebrate marine masses, crabs exhibited CRABS 

 considerable development. Swimming crabs (Portunites), 

 long-beaked spider-crabs (Oxyrhyncha), sharp-nosed crabs 

 (Matuta), and crabs in carapaces shaped like that of the 

 modern land-crab (Catometopa) were all in evidence. Hermit- 

 crabs classed, on account of their long tails, with lobsters 

 had also made their appearance (Paguridai). 



A remnant of Ammonites lived on, but their annals soon CEPHALOPODS 

 closed. Belemnites were represented only by a few forms 

 with the internal shell much reduced in size. Some of them 

 seem to have been becoming very much like the ordinary 

 sepia or cuttle-fish of our own times (Belosepia). Others 

 resembled those modern cuttle-fishes in which the shell has 

 almost disappeared (Spirula). Nautili continued prosperous, 

 and were thus restored, after a long eclipse, to a position of 

 importance among cephalopods. 



Gastropods were assuming a modern aspect. Siphoned UNIVALVED 

 forms, which had been improving their position ever since MOLLUSCS 

 the Trias, were now the prevailing type whelks (Buccinum), 

 cowries (Cyprcea), mitre-shells (Mitra), and olive-shells 

 (Olivd) being in great force. Some forms were living in 

 turreted shells half a yard in length (Ceritheum). Old- 

 fashioned siphonless sea-snails, although relatively quite 

 unimportant, were well represented by some old genera that 

 had held on from the Trias (Natica, etc.). 



Chitons were now of modern type, with the shell plates 

 well attached to the margin of the mantle (Ischnochitonida). 



Many genera of bivalved molluscs had now been super- BIVALVED 

 seded by modified forms. Existing genera of mussels (Ano- MOLLUSCS 

 donta), sponge-borers (Vulsella), and various burrowing bivalves 

 had become distinct (Cytherea, etc.) : and clams approximated 

 much more to living forms (Byssocardium, Lithocardium). 



Lampshells, so far at least as regards reduction of families, LAMPSHELLS 

 now rested on the bed-rock of their depression. 



