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885 



TERTIARY FAUNA OF FLORIDA "^ 



Museum were obtained at Bombay. They are evenly rounded at both ends 

 and have the texture and surface of Raeta. 



Genus TRESUS Gray, Jan. i, 1S53. 

 Type T. Nuttallii QomKA, 1837. 



= Cryptodoii Conrad, 1837, not Turton, 1822; -|- SchhotJiceriis Conrad, Jan. 31, 1853; 

 + Trcsus Dall, Naut. , viii. , 42, i S94. 



Shell large, inequilateral, thin, inflated ; siphonal gape very large, pedal 

 gape narrow ; ligament minutely sagittate, separated by a shelly lamina from 

 the pit, which lamina is often recurved and patulous ; resilium homogeneously 

 continuous between the valves ; left cardinal high, compressed, with a strong 

 posterior accessory lamella roofing the apex of and projecting over the pit ; 

 laterals small, but distinct in both valves ; right cardinal feeble, not coalescent 

 above, the anterior arm superposed upon the ventral lamina, posterior arm 

 walling and overhanging the pit. Gills, foot, palpi, and mantle-margin not 

 differing in any essential particular from those of Spisula siniilis Say. 



The siphons are large, united to the tips, firmly clothed with a coarse 

 epidermis ; siphonal orifices papillose ; the end of the united siphons when 

 the papillose tips are retracted shuts like a book instead of contracting circu- 

 larly, and the horny epidermis accumulates with growth on the flat lateral 

 portions which correspond to the covers of the book and forms flattish masses 

 which sometimes resemble " horny valves," as described by Conrad, but which 

 are a purely mechanical product not comparable to the "pallets" of Teredo. 

 Northern specimens have the siphonal tunic more rugose and the siphonal 

 " valves" less clearly formed than in the southern ones. Something of the 

 same sort can be observed at the ends of the siphons in Mya truncata, Platyo- 

 don cancellatns, and, doubtless, in other burrowers with long tunicated siphons. 



The chief feature in which T/Ti'zw differs from the Spisula referred to, 

 apart from its permanently exserted siphons and slightly less open mantle, is 

 the great development of a thin membrane behind and extending from the 

 siphonal septum towards the gills, to which it is attached. In Spisula the edges 

 of the gills are closely adjacent to the siphonal septum with little membrane 

 intervening. The gills in Trcsus are more coarsely plicate than in Spisida 

 and proportionately somewhat smaller. The osphradial raphe which bisects 

 the current from the branchial siphon in Spisula is less prominent and more 

 ventrally situated in Tresus. In other respects the gross anatomy did not 

 differ more than one would expect to find in two species of the same genus. 



