TRANSACTIONS OF WAGNER 

 876 



TERTIARY FAUNA OF FLORIDA 



acters like Mactrodcnna. Distribution, Australian seas and New Zealand. 

 M. tristis Gray may serve as type. 



Subgenus MACTROTOMA Dall. 

 Mactrotoma Dall, Nautilus, viii., p. 26, 1S94. Type M. fragilis QvufXiw. 



Shell subequilateral, elongate; with a thin, silky epidermis, posterior 

 dorsal areas bordered by an impressed fascicle over which the epidermis is 

 darker colored and differently wrinkled ; beaks adjacent ; pallial sinus large; 

 valves convex, gaping markedly ; ligament lanceolate ; chondrophore large, 

 shallow, apically roofed ; anterior lamina issuing from the dorsal sinus ; 

 cardinals prominent, thin, their posterior arms projecting over the chondro- 

 phore ; each anterior arm attended by a high accessory lamella in nearly the 

 same plane, closely appressed in the right valve to the ventral lamina, and in 

 the left valve to the anterior lateral, so that, to a cursory inspection, the lamina 

 appears tridentate and the tooth bidentate. (See pi. 27, figs. 1,4, 8, 18.) 



This group is widely distributed over the world in the tropics, usually a 

 single species in each fauna. The type is better known as M. brasiliana Lam. 

 These species have been confounded with Siandella Gray, which was based 

 on M. pellucida Chemn. (which Gray by an error referred to z.sfragilis Chemn., 

 another species figured on the same plate of the Conch. Cabinet) and M. 

 cEgyptiaca Dillwyn, species belonging to the Lutrariina:. Gray's error of 

 reference is explained by manuscript notes of Deshayes in the writer's posses- 

 sion and by specimens identified after Gray by Cuming for the United States 

 National Museum ; but the details make too long a story to recapitulate here. 

 The soft parts of this group (as might be anticipated from the shell characters) 

 show modifications pointing towards Ltitraria. Standella has a spisuloid 

 ligament. 



Section Simomactra Dall. 

 Nautilus, viii., p. 40, 1894. Type M. dolabriformis Conrad, Gulf of California. 



Shell inequilateral, flattened cuneiform ; pallial sinus smaller ; siphonal 

 gape inconspicuous ; accessory lamellae distant from the laterals ; otherwise 

 as in Mactrotoma s. s. 



Section Micromactra Dall. 

 Nautilus, viii., p. 40, 1894. Type M. californica Conrad, non Deshayes. 



Shell small, solid ; hinge like Mactrotoma s. s. ; beaks sulcate. (See pi. 28, 

 fig. 12, and pi. 37, fig. 23.) 



