[47]

The following letters of reference apply to all the figures of Coronula: а, sheath marked transversely in the upper part by the attachment of the

opercular membrane a ' , ala б, furrow on each side of (a), receiving the edge of the thick ala of the

adjoining lateral compartment c, special plate, on which the ala rests d , radius, on the edge it may be just seen to consist of an outer layer (the

normal radius), and a much thicker inner part (the pseudo or comple­ mentary radius) formed of oblique denticulated septa

e e ' e", basal edge of wall, which from its commencement at e, or e '\ can be followed, folding up to near the basal edge of the sheath, to its termination at e" or e

f /, serrated lines of junction between the folds of the wall

Fig. ga, Verruca Strõmia, much enlarged, engraved from a recent specimen, only certain valves having been found fossil

96, Verruca Strõmia, fixed scutum and tergum, internal views. The following letters apply to both these figures: a, rostrum B, carina s, movable scutum, s', scutum fixed and modified so as to form part of shell T, movable tergum, t ', tergum fixed, forming part of shell

[48] W O R K S OF d a r w i n : v o l u m e 14 In s, and s', a is the occludent margin; the basal margin; m, the plate to

which the adductor muscle is fixed In s and s', the tergal margin is marked by small dashes; (') being the upper

articular ridge, and (") the second or lower articular ridge: in s' (") is called the parietal portion of the valve

In T and t ' , the scutal margin is marked by small dashes: ( ') being the first and upper articular ridge, hardly distinct from the occludent margin, and called in t ' the occludent rim; ( " ) is the second, or middle, and ( ' " ) the lower or third articular ridge, called in t ' the parietal portion of the valve: x is the carinal margin, called in t ' the carinal rim, and z the basal margin

Fig. 10a, Verruca prisca, five times nat. size 106, 10c, scutum and tergum, external views; engraved from a

Belgian cretacean specimen, the opercular valves not having been found in England. /