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TERTIARY FAUNA OF FLORIDA 



Rrachidonlcs Swainson, Malac., p. 384, 1840. Type Moitin/u snlcata Lam. 



Moiliflla Monterosato, Noiu. Con. Medit., p. 12, 1884 (not of Hall, 1883). Type Modiola 



pulita Yerrill. 



Grcgariclla. Monterosato, op. cit., p. 11, 1884. Type Mytilus petagnce Scacchi. 

 Rrachydiwtes Fischer, Man. de Conch., p. 968, 1886; Dall, Bull. U. S. Nat. Mus., No. 



37, p. 38, 1889. 



The main conchological characters which separate this genus from Mytilus 

 are the non-terminal umboncs, the tendency to hirsuteness in the epidermis, 

 the absence of developed teeth at the beaks, and the habit of nestling in a 

 mass of byssal fibres with extraneous entangled material which is more or less 

 characteristic of all true Modioli, though less conspicuous and complete in the 

 larger species. In some deep-water species a real nest is spun, like that of 

 Lima, but more dense. A more efficient protective device could hardly be 

 imagined than the byssal nest of M. polihts, which completely conceals the 

 occupant from predacious marine carnivora. In the matter of sculpture these 

 shells resemble Mytilus, and have a distinct tendency to a medial unsculptured 

 area in the radially sculptured species. Although true teeth are not found in 

 this group, the provinculum is often present and permanent, while its origin is 

 obviously indicated by the secondary denticulations due to the impinging of 

 the radial sculpture upon the margin. 



I believe hinge-teeth were thus originally initiated, while the secondary 

 denticulations alluded to repeat in the descendants the process by which their 

 remote ancestors acquired an interlocking hinge. 



The genus Modiolus, like Mytilus, may be divided into natural groups by 

 the sculpture of the surface. 



Genus Modiolus Lam. 



Section Modiolus s. s. Surface smooth, shell inflated, edentulous, epi- 

 dermis more or less hirsute. Type M. modiolus Linne. 



Section Amygdalum Megerle. Surface smooth, shell compressed, epidermis 

 polished, not hirsute. Type M. picttts Lam. (Syn. Modiclla Mts., not Hall.) 



Section Grcgariclla Mts. Surface decussate with a central smooth area; 

 shell plump, epidermis hirsute. Type M. petagnic Scacchi. (Syn. Botnlina 

 Dall, 1889.) 



Section Brachydontcs Swainson. Surface more or less radially sulcate ; 

 epidermis not hirsute. Type M. sulcatits Lam. (1819, not 1807). Scini- 

 mvdiola and Wanitnodiola Cossmann seem to belong to this section rather 

 than to Modiolaria. 



