208 INVERTEBRATE PALAEONTOLOGY 



species of that genus, accompanied by the end-forms of 

 the Clydonautilidae (Aturia\ occur in the London Clay. 

 Ammonoids are altogether wanting, but degenerate 

 Belemnoids (e.g. Beloptera\ with insignificant guards, 

 lingered into the Eocene. The recent Spirula, with a 

 coiled, almost " Gyroceracone," shell, seems to show 

 development of long-deferred enrolment in this usually 

 " Orthoceracone " group. Cuttle-fish and Octopods 

 occur sparingly. The majority of the latter are devoid 

 of hard-parts, and their frequently gigantic proportions 

 suggest phylogerontic over-development comparable 

 with that of the Elephants. 



(K) ARTHROPODA 



Little comment is needed on Cainozoic Arthropods. 

 The prevalent orders are those dominant at the present 

 day ; a large proportion of genera have persisted from 

 the Eocene. The common "acorn-barnacle" (Balanus) 

 first appeared in the Eocene; it is abundant in the 

 Pliocene Crags. Crabs and Lobsters are common in 

 the London Clay, especially in the Isle of Sheppey. 

 Hoploparia and Xanthopsis (PI. xvi. fig. n) are well- 

 known representatives of the two series. More delicate 

 Arthropoda, such as Myriopoda and Insecta, are best 

 known by forms enclosed in amber (usually of Oligocene 

 date); but an abundant and wonderfully preserved 

 insect-fauna has been discovered in fresh-water marls 

 at Florissant, Colorado. Lepidoptera, Diptera and 

 Hymenoptera of essentially modern aspect, often retain- 

 ing much of their coloration, have been found in that 

 remarkable deposit. Specimens enclosed in amber are 

 normally perfect and uncompressed. Knowledge of 

 small forms such as the Diptera is made accessible by 

 such a mode of preservation. 



