504 VERRUC1DJ5. 



side of the adductor plate of the fixed scutum, to liga- 

 mentous fibres, presently to be mentioned, by which the 

 shell is attached to the basal membrane : the carina and 

 rostrum being so much more gently inclined, do not stand 

 in need of a ledge for their attachment. 



By comparing the moveable scutum and tergum with the 

 corresponding fixed valves, in all the species, the modification 

 of the latter may be clearly made out to have been effected 

 as follows ; and the case appears to me a striking and inter- 

 esting one. The moveable scutum and tergum lie in the 

 same plane, and are articulated by the means of three ridges 

 on the tergum (including the occludent margin), and by two 

 on the scutum. The fixed scutum and tergum have to be 

 curved, and to be greatly increased in size ; and this is 

 brought about, as we shall see, by the large development of 

 a certain small portion of each valve. Comparing first the 

 moveable tergum (t) with the fixed tergum (t'), the umbo 

 of growth matches the umbo : of the four margins of the 

 moveable valve, the carinal (of), basal (z), and occludent 

 (p in fig. 5), margins can be identified with certainty in the 

 fixed valve, from their close similarity in shape, their abso- 

 lute apposition, or correspondence in position. There 

 remains only the scutal or articular margin, with its three 

 articular ridges ; of these, the uppermost ('), inasmuch as 

 in most of the species it is hardly distinct from the oc- 

 cludent margin, can, as we have just seen, be clearly 

 identified, and is overlapped, as it normally should be, by 

 the upper tergal corner of the fixed scutum : the second or 

 middle articular ridge, though not so distinct as in the 

 moveable valves, can be plainly recognised ("), t', in fig. 1 b, 

 and 5 ; and it serves its normal function of articulating the 

 two valves together. But when we look in the fixed valve 

 for the third or axial ridge ('"), we find in its exact place, 

 namely, extending from the umbo to the extreme opposite 

 end of the valve, between the second articular ridge (") and 

 the basal margin {z, see fig. 5), only that portion of the valve 

 which I have called the parietal portion ; consequently, I do 

 not doubt that this really is the axial ridge largely expanded. 

 So again in comparing the moveable scutum (s) with the fixed 

 scutum (s') ; two of the three margins of the former, namely, 



