262 MINERALS AND GEOLOGY 



amples have not been detected in our strata, although found in the 

 Palaeozoic rocks of the United States. 



7. Amphipoda : This Order (which may be made to include the 

 Laemodipoda of some systems of classification) comprises a series of 

 small shrimp-like crustaceans, represented by " water-fleas," " sand 

 hoppers," " spectre-shrimps," &c., with sessile eyes and typically seven 

 pairs of legs, the front pairs directed backwards, and the hind pairs 

 forwards, whence the name of the group. Marine, fresh -water, and 

 terrestrial types are known. Fossil forms (Gampsonyx, &c.) only 

 date with certainty from the Carboniferous period, and the amphipods 

 are unrepresented in our strata. 



8. Isopoda : The isopods are small crustaceans, very similar in 

 character to the amphipods, and including, like the latter, marine, 

 fresh- water, and terrestrial types ; but the form instead of being 

 laterally compressed, is generally flattened from above downwards. 

 The Oniscus or " wood-louse " is the typical terrestrial representative. 

 Fossil forms of the group, date from the Devonian period, but are of 

 comparatively little interest. 



9. Stomapoda: This division comprises small shrimp-like crus- 

 taceans with stalked eyes, and with branchiae suspended from the 

 abdomen or attached to the thoracic feet. Squilla and Mysis are 

 typical examples. Species of the latter are familiarly known as 

 " opossum shrimps." Fossil forms date from the Jurassic period, but 

 are rare and of no special interest. 



10. Decapoda : In the Decapods the type-forms of the Crus- 

 tacea the true feet are always in five pairs ; the eyes are stalked ; 

 the head and thorax are united into a cephalo- thorax ; and the 

 branchiae are in special cavities at the sides of the latter. Three 

 leading groups are recognized : (1) Macrura or long-tailed decapods, 

 in which the abdomen is well-devoleped, forming a powerful swim- 

 ming organ. Typical representatives comprise lobsters, cray-fish, and 

 true shrimps. Fossil forms date from the Carboniferous period* 

 (2) Anomura, or defenceless-tailed decapods, in which the abdomen 

 is unprotected by a shelly covering, and does not serve as a natatory 

 organ. The Paguridae or " Hermit-crabs " are examples. These 

 insert the abdomen into the vacant shells of whelks or other gastero- 

 pods, or keep it buried in the sand of the sea shores on which they 

 live. Fossil forms date from the Jurassic period. (3) Brachyura 



