CRUSTACEA. CIRRIPEDIA 317 



Distribution of the Cirripedia 



The Cirripedes are all marine, and the greater number 

 are found in shallow water, particularly near the coasts, 

 Balanus being especially characteristic of littoral regions. 

 At depths greater than 1000 fathoms, only two genera, 

 Scalpellum and Verrucosa, have been found, and these are 

 not confined to deep water. 



Cirripedes are rare in the Palaeozoic and early Mesozoic 

 formations, but become moderately common in the Chalk, 

 and are abundant in some of the later Tertiary deposits. 

 A few examples, which are believed to be Cirripedes, 

 have been found in the Cambrian of North America. In 

 the Ordovician the genera Pollicipes, Scalpellum, and 

 Turrilepas occur : the first two are represented in the 

 Silurian and various later formations (especially the 

 Chalk) ; the last ranges on to the Devonian. Loricula 

 and Brachylepas are found in the Chalk. Balanus appears 

 in the Eocene, and Lepas in the Pliocene. 



SUB-CLASS VI. MALACOSTRACA 



The Malacostraca are usually of larger size than the 

 Crustacea belonging to the preceding groups. With the 

 exception of the Leptostraca, the number of segments is 

 constant, there being eight in the thorax, and seven in 

 the abdomen (including the telson), making altogether 

 twenty segments in the body. The abdomen is clearly 

 marked off from the thorax by the character of the 

 appendages. In some cases the development is direct, 

 the young having the same or nearly the same form as 

 the parent, but usually larval stages occur ; the principal 



