192 CRUSTACEA. DECAPODA. 



usually strong cephalothoracic shield ; this is frequently 

 marked out into anterior and posterior portions by a 

 groove, the cervical suture, the anterior belonging to the 

 head, the posterior to the thorax. Anteriorly the shield 

 is produced into a spine, the rostrum. The gills are placed 

 in a chamber formed by the downward prolongation of 

 the cephalothoracic shield. The cephalothorax bears 

 thirteen pairs of appendages, five belonging to the head, 

 eight to the thorax. On the head there are (1) antennules, 

 (2) antennae, (3) mandibles, (4 and 5) maxilla, the last 

 three pairs serving as jaws. On the thorax the three first 

 pairs (maxillipedes) serve in locomotion and also in masti- 

 cation ; the posterior five pairs are the ambulatory limbs, 

 the first pair being the large chelae. The abdomen bears 

 six or fewer pairs of small appendages. 



There are two sections, (1) Macrura, (2) Brachyura. 



Section 1. Macrura. 



This includes the lobsters and crayfishes. The abdo- 

 men is long, well developed, and provided with four or 

 five pairs of appendages. There is a large caudal fin. 



Meyeria. Cephalothorax laterally compressed, with 

 a sharp rostrum, and a deep V-shaped cervical suture; 

 the posterior part of the cephalothorax covered with sharp 

 granules. Abdomen semicylindrical, longer than the 

 cephalothorax, and ornamented with rows of granules. 

 Lower Greensand. 



Section 2. Brachyura. 



This includes the crabs. The abdomen is short and 

 more or less rudimentary ; it is curved up underneath the 

 thorax, and bears from one to four pairs of appendages. 

 There is no caudal fin. 



