8 INTRODUCTION. 



properly the anterior end, and the tips of the terga, often 

 hidden within the shell, are properly the posterior end of 

 the external covering ; bnt I have found it more convenient 

 to speak of the upper and basal surfaces and aspects, which 

 hardly admit of any mistake. A line drawn from the centre 

 of the basis, along the middle of the rostrum to the tips of 

 the scuta, shows the strictly medio-ventral surface of the 

 shell ; and another line drawn from the centre of the basis, 

 along the carina, to the tips of the terga, shows the strictly 

 medio-dorsal line ; but from the crooked course of these 

 lines, I have found it far more convenient to speak of the 

 rostral and carinal end or aspect of the different parts of 

 the shell ; this is the more necessary with respect to the in- 

 ternal parts of the animal, owing to their remarkable changes 

 of position during the metamorphosis, whence it comes 

 that the dorsal surface of the thorax faces partly dorsally, 

 partly anteriorly or downwards, and partly even ventrally ; 

 and the ventral surface of the whole posterior part of the 

 thorax faces upwards or posteriorly ; but when we refer 

 these parts to the rostral, carinal, basal, and upper ends of 

 the shell, there can be no mistake. There has moreover 

 been great confusion in these relative terms, as applied by 

 different authors. 



When a sessile Cirripede is held in the position in which 

 they have generally been figured, namely with the basis 

 downwards and the scuta towards the beholder, then the 

 right and left sides of the Cirripede correspond with those 

 of the holder. 



I have followed the example of Botanists, and added the 

 interjection [!] to synonyms, w T hen I have seen an authentic 

 specimen bearing the name in question. 



Every locality, under each species, is given from spe- 

 cimens ticketed in a manner and under circumstances ap- 

 pearing to me worthy of confidence, — the specific deter- 

 mination being in each case made by myself. 



