92 AKTICULATA. 



CLASS I. — CRUSTACEA. 



la this class the skeleton has the form of an external crust or shell, 

 which covers even the antennas, hairs, jaws, and teeth. In the smaller 

 species it is chitinous ; in others it is hardened hy the carbonate and 

 phosphate of lime. The normal number of segments is 21, for the head, 

 thorax, and abdomen, seven each. But most frequently the anterior 

 segments form one piece called the cephalothorax, leaving the abdomen 

 joined, terminated- by a compound tail-piece, pygidium. This heteronamity , 

 or the having no generally equal number of body-rings, is the most 

 essential class characteristic. All Crustaceans are organized for life in 

 the water, though many live chiefly on land. The earliest forms were 

 Trilobites. 



Nearly all the other fossil Crustaceans are Decapods. The oldest 

 approach nearest to the existing tropical species ; while the more modern 

 ones have a greater resemblance to temperate species. Those found 

 fossil in the tropics are more akin to the Crustacea now found in the 

 same latitude. In general we may say that the Paleozoic Trilobites are 

 followed by Lobsters in the -Jurassic Period, and by Crabs in the Terti- 

 ary. The Upper Oolite of Solenhofen opens like a book filled with com- 

 pressed and wonderfully preserved Shrimps and Lobsters, while the 

 London Clay of the Isle of Sheppey is a rich repository of short-tailed 

 Crustaceans. The Articulates which came latest are the Cirripedes, 

 whose lowest species (Pollipeds) appears in the Lias. The fossil forms 

 (of which 69 have been described) belong chiefly to the sessile division. 



Saits-Class IS.— Malacostraca. 



The section Malacostraca includes nearly all the large, and many of 

 the smaller species with which we are most familiar. They have either 

 ten or fourteen feet; their body is divided into thorax and abdomen with 

 seven segments in each ; the Decapods (Crabs, Lobsters and Shrimps) 

 being the highest of the class. The Macrourans abound in the Oolite 

 and Cretaceous strata, while the Brachyourans attain their maximum 

 development in Tertiary beds and modern seas. 



No. 349. Lobocarcinus Paulo- W"urtemburgensis, Meyer. 



This interesting and well-preserved fossil Crab 

 occcurs in considerable numbers in the Lower Ter- 

 tiary (Nummulitic) beds of the Gebel Mokattani in 

 the suburbs of Cairo, Egypt. Original in the Ward 

 Museum of the University of Rochester. 



Size, fi x 4 in. Price, $1.50. 



