INTRODUCTION. Te 
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 refexed. On the internal surface of the valve there is almost 
always an adductor pit or cavity (fossa adductoris), 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 dateral-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 
carinal ; its upper end, or apez, is sometimes deaked ; on the basal margin a spur (calcar) 
depends; the outer surface of the valve is depressed or longitudinally furrowed (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. 
Relative position of parts.—The centre of the generally flat basis, which is cemented 
to the supporting surface, is properly the azterior 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 wpper and dasal 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 m 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. 
