INTRODUCTION. 11 



During the periodical growth of the valves, especially when they are thick and massive, 

 it happens in several species that the underlying corium deserts their upper ends or umbones, 

 which consequently become marked by lines or ridges of growth, as I have called them, 

 though perhaps lines of recession would have been more strictly correct. Such valves, 

 consequently, have their upper ends projecting from and beyond the capitulum, and are 

 said to project freely or lihere ; this is often more especially the case with the Carina in 

 PoUicipes, and in a lesser degree with the Terga. 



From the peculiar curved position which the animal's body occupies within the 

 capitulum, I have found it far more convenient (not to mention the confusion of nomen- 

 clature already existing) to apply the term Rostral instead of ventral, and Carinal instead of 

 dorsal, to almost all the external and internal parts of the animal. Cirripedes have generally 

 been figured with their surfaces of attachment downwards, hence I have termed the lower 

 margins and angles the Basal, and those pointing in an opposite dii'ection the Upper; 

 strictly speaking, the exact centre of the usually broad and flat surface of attachment is 

 the anterior end of the animal, and the upper tips of the Terga, the posterior end of that 

 part of the animal which is externally visible ; but in some cases, for instance in Coronula, 

 where the base is deejjly concave, and where the width of the shell far exceeds the depth, 

 it seemed almost ridiculous to call this, the anterior extremity ; as likewise does it in 

 Balanus to call the united tips of the Terga, lying deeply within the shell, the most pos- 

 terior point of the animal as seen externally. 



