6 INTRODUCTION. 



The walls of the shell, the basis, and the radii, are in very 

 many cases composed of an outer and inner lamina, united 

 together by longitudinal septa ; a set of tubes or pores being 

 thus formed. The points of the longitudinal septa gene- 

 rally project beyond the laminae, and are denticulated on 

 both sides (see woodcut, further on ;) the septa are some- 

 times branched, several irregular rows of pores between the 

 two laminae being then formed (see PL 7, fig. 3 b, and 

 PL 10, fig. \g, 1//). 



Operculum, or opercular valves. — These consist of a pair 

 of scuta and a pair of terga. They are joined to the sheath 

 of the shell by the opercular membrane. 



Scutum (woodcut 5) : this valve is generally sub-trian- 

 gular, and its three margins are the basal, the tergal, so 

 called from being articulated with the tergum, and the 

 occludent, so called from opening and shutting against the 

 opposed valve. The angles are called from the adjoining 

 margins, as basi-tergal, &c. ; the upper angle being the apex. 

 The scutum is ordinarily articulated to the tergum by an 

 articular ridge [crista articularis), running up to the apex 

 of the valve, and by an articular f arrow, which latter receives 

 the scutal margin of the tergum. The articular ridge, in- 

 stead of projecting straight up from the valve, when laid flat 

 on its external surface, often bends over to the tergal side, 

 and is then said to be reflecced. On the internal surface of 

 the valve, there is almost always an adductor pit or cavity 

 [fossa adductor is), for the attachment of the adductor scu- 

 torum muscle : this pit is often bounded on its tergal and 

 basal sides, by a ridge, called the adductor ridge [crista ad- 

 duct oris), 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 later nl-de- 

 pressor pit [fossa musculi lateralis depressoris), for the at- 

 tachment of the so-called muscle ; and this pit is some- 

 times furnished with crests. 



Tergum, (woodcut G 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 furrowed [sulcus longiludiualis) in the line 



