3o6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



all of these is Balanus balanoides which grows in such immense 

 numbers over the whole northern Atlantic coast that it covers 

 the rocks in many places with a white incrustation. 



1. Distinguish between the calcareous skeleton of the acorn 

 barnacle and the skeleton of the goose barnacle. Give their 

 geologic range. 



2. Briefly trace the development of a barnacle. 



3. Compared with its own larval state, is the adult advanced 

 or degenerate ? Why ? 



4. What is the habitat of living barnacles ? 



Sub-class 6, Malacostraca 



Body usually divided into a thorax of eight and an abdomen 

 of seven segments. A gastric mill for grinding the food is 

 present. Excretory organs are glands in the antennae. The 

 nauplius stage is usually passed through in the egg. Known in 

 the fossil state from the Cambrian to the present. 



Derivation of name. — Greek malakos, soft, + ostrakon, shell; 

 so named because the carapace or shell is softer than the shell 

 of the mollusk. 



This is subdivided into the following orders : 



The old division Entomostraca, comprising the very diverse 

 groups Phyllopoda, Ostracoda, Copepoda and Cirripedia, was 

 characterized by only negative characters, while the old order 

 Schizopoda comprised the three very distinct groups, — Anas- 

 pidacea, Mysidacea and Euphausiacea. 



(a) Phyllocarida (Cambrian to present), a connecting link 

 between the Phyllopoda and Copepoda on the one hand and 

 orders d-f on the other. Example, Ceratiocaris (Ordovician 

 and Silurian), {b) Anaspidacea (Pennsylvanian to present). 

 A well-known example is Paleocaris from the Pennsylvanian 

 coal measures of North America and England, (c) Mysidacea 

 (Mississippian to present), (d) Cumacea (not known fossil). 

 {e) Isopoda (Devonian to present). A common living example 

 is the pill-bug (Armadillidium) ; this is found in damp places 



