SPONDYLUS. 



account of the Genus Ostrea, and we here repeat it, to 

 show how impossible it is that the animal should displace 

 its upper valve, as Lamarck asserts, in order to produce 

 the progressive elongation of the area of the hinge of the 

 lower valve. The ligament in the present Genus is dou- 

 ble ; the principal portion being internal, turgid and fixed 

 in the central pits of the two hinges : the other portion is 

 external, slender, and attaches the valves to each other 

 along the linear margin of each valve : this seems hitherto 

 to have escaped the notice of conchological writers, but it 

 is, nevertheless, distinct in this genus, as well as in Pectetiy 

 the genus to which Spondylus approaches nearest in na- 

 tural affinity. A single, suborbicular, rather lateral mus- 

 cular impression is observable, and the impression of the 

 muscle of attachment of the mantle is continuous, sur- 

 rounding the former. 



Because of the varied forms which the spines and 

 foliations of the outside take, it is extremely difficult to 

 distinguish and describe the species in this fine Genus ; 

 Lamarck has published the specific characters of twenty- 

 one recent species ; we venture to think, however, that se- 

 veral of these ought to be regarded only as variations : one 

 of the most distinct and beautiful of them is the Sp. regius, 

 remarkable among its congeners for the regularity of its 

 form, the length of its spines and the smallness of the car- 

 dinal area: next to that we think the Sp. aurantius a re- 

 markably beautiful shell: the Sp. ducalis is worthy of par- 

 ticular notice for the richness and variety of its colours : 

 they are all extremely ornamental and consequently much 

 valued by collectors. Besides the recent, Lamarck de- 

 scribes four fossil species, all of which appear to belong 

 to the more recent formations : we have represented one 

 which we believe to be Lamarck's Sp. podopsideus. 



Fig. 1. Spondylus aurantius^ inside of the upper valve, to shew the hinge. 



2. , inside of the lower valve to show the hinge and 



the cardinal area when the ligamental groove is closed. 

 S. Hinge of the lower valve of Sp. Gcedaropus to show the cardinal area 



with its ligamental groove open. 



4. Sp. ducalis. 



5, Sp. podopsideus. 



