OPERCULAR VALVES. 53 



furrow in the scutum has its further border generally pro- 

 minent and often reflexed or curved over; I have called it 

 the articular ridge; it, also, is lodged in a furrow in the 

 upper part of the tergum, which again is bordered by a 

 ridge, viz., the articular tergal ridge. So that in both 

 scutum and tergum there is an articular furrow, bordered 

 in each case, on one side by the margin of the valve, and on 

 the other side by the so-called articular ridge. In Chelonobia 

 (PI. 14, fig. 1 b) the articular ridge of the scutum is horny. 

 When, as often happens, the scuta and terga have been 

 much worn, the manner of their articulation (PL 18, fig. 1 a) 

 is pretty well shown even from the outside ; in this case 

 their external appearance is very different from what it is 

 in those individuals (fig. 1 c) of the same species^ which 

 have not suffered disintegration. This articulation of the 

 scuta and terga is prefigured amongst the Lepadidse, in 

 Pollicipes mi fella, and in Lithotrya. 



The scuta are brought together by a short, strong, 

 straight, adductor muscle (PL 25, fig. 1 a) ; its attachment 

 leaves (with very few exceptions, as in Tubicinella) a 

 rounded impression, or even pit, on the under side of the 

 valve in its upper part. This pit is frequently bounded, on 

 its lower side, by a sharp ridge, which, though not in actual 

 connexion with the adductor muscle, I have, for convenience 

 sake, called the adductor ridge ; it serves apparently to give 

 support to the animal's body; in some few cases (as in 

 B. psittacus, PL 2, fig. 3 e) it is confluent at its upper 

 end with the articular ridge, and converts the whole basi- 

 tergal corner of the valve into a deep cavity. In some of 

 the species of Pyrgoma (PL 12, fig. 5 c, 7 5), and in some 

 varieties of Creusia, this adductor ridge is enormously deve- 

 loped, so as to depend far beneath the true basal margin, 

 or that to which the opercular membrane is attached. At 

 the basi-tergal corner of the valve, there is generally a small 

 pit or impression, and sometimes distinct crests, for the 

 attachment of the lateral depressor muscle. At the rostral 

 end there is, also, a small cavity formed by the overfolcling 

 of the occludent margin (rarely furnished with crests) for the 

 attachment of the rostral depressor muscle. In the Terga, 

 at the basi-carinal corner, there are usually crests, though 



