INTRODUCTION. 1 1 



scutal margin of the tergum. The articular ridge, instead of projecting straight up from 

 the valve, when the latter is laid flat on its external surface, often bends over to the tergal 

 side, and is then said to be rejlexed. On the internal surface of the valve there is almost 

 always an adductor pit or cavity {fossa addi/ctoris), for the attachment of the adductor 

 scutorum muscle : this pit is often bounded on its tergal and basal sides, by a ridge, called 

 the adductor ridge {crista adductoris), which, in its upper part, is often confluent with the 

 articular ridge. Beneath the adductor ridge, in the basi-tergal corner of the valve, there 

 is often a lateral-depressor pit {fossa musculi lateralis depressoris), for the attachment of 

 the so-called muscle ; and this pit is sometimes furnished with crests. 



Tergum, (figs. 6 and 7) : — this valve, also, has three margins, the scutal, basal, and 

 carina! ; its upper end, or apex, is sometimes beaked ; on the basal margin a spur {calcar) 

 depends ; the outer surface of the valve is depressed or longitudinally furroioed {sulcus 

 longitudinalis) in the line of the spur. The part called the spur is often so broad, that the 

 name becomes not very appropriate. The angles are denominated, from the adjoining 

 margins, as basi-carinal, or basi-scutal angle, &c. On the under side, in the upper part, 

 there is an articular ridge, and on its scutal side, an articular furrow, receiving the 

 articular ridge of the scutum. In the basi-carinal corner of the valve there are often crests 

 for the attachment of the tergal depressor muscle. 



Belative position of parts. — The centre of the generally flat basis, which is cemented 

 to the supporting surface, is properly the anterior end, and the tips of the terga, often 

 hidden within the shell, are properly the posterior end of the external covering; but 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 scula, 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. 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. 



