90 



Genus 5. MYCETOPUS, HOrbigny. 



Animal; with the mantle-lobes disunited; siphoned tubes none; 

 foot very long, cylindrical, produced and inflated into a knob at 

 the extremity. 



Shell ; thin, much elongated, cylindrical, gaping at each end, par- 

 ticularly at the anterior end ; umboes central ; hinge linear, 

 toothless ; ligament marginal. 



A Naiad of very distinct typical character from the rest of the family, of 

 which three species were discovered in the Bolivian Republic of South 

 America by M. D'Orbigny, and a fourth has been found there since. The 

 peculiarities of the animal are that it has no siphonal tubes or orifices, the 

 mantle-margins being entirely free, and it has an elongated club-like foot, 

 which does not appear to be drawn within the shell. This supposition is 

 moreover favoured by the, conspicuous gaping of the shell at its front ex- 

 tremity. 



Species. 



1. siliqnosus, D'Orb. 3. ventricosus, D'Orb. 4. Weddellii, Poey. 



2. Soleniformis, D'Orb. 



Figure. 



Mycetopus Soleniformis. PI. 32. Shell, showing the lateral and ven- 

 tral sinus with gaping extremities. 



Family 3. TRIGONACEA. 



Genus 1. TRIGONIA, Bruguiere. 



Animal ; shaped like the shell, with the mantle-lobes disunited 

 about three-fourths of their extent, and rather thickened and 

 crimped at the edge ; foot long, and sharply angled, bordered 

 by two crenulated ridges. 



Shell ; equivalve, obliquely triangularly ovate, lined internally with 

 lustrous iridescent nacre ; hinge composed of two oblong divari- 

 cate lamelliform teeth in one valve, regularly striately grooved 

 on both sides, interlocking with three to four teeth in the other 

 valve, grooved on one side only ; ligament external, marginal. 



