HORN EXPEDITION—MOLLUSCA. 
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The structural details now given' are not reconcilable with the accepted 
position of this genus in Rhytididse, and 1 apprehend that it should correctly 
appear in the Endodontidte, intermediate between Laoma and Flanunulina. 
The very short transverse rows of the radula, compared with that of the 
average Endodontida^. suggest specific degeneration in size from a larger ancestor. 
The diminution of the radula having been effected by reducing the number rather 
than the size of the individual teeth, as the latter process would have rendered it a 
less serviceable instrument. 
Thersites setigera, Tate. 
Several specimens from Palm Creek were 
dissected. The jaw (Fig. D) is stout, arcuate, 
with fourteen broad strong ribs denticulating 
both margins, and sepai’ated by very narrow 
interstices. 
In the odontophore (Fig. E) were counted 137 transverse rows of 18 ; 16 : 1 : 
16 : 18. The rachidian is reflected in a stout ovate cusp, the length of the basal 
plate; laterals a single broad blunt cusp overlapping the basal plate and slightly 
inclined to the rachidian. In the transition teeth the basal plate shrinks in 
size, the ectocone hrst develops, then the mesocone exhibits an emargination 
which buds into an ectocone cusp. Marginals trifid. 
As regards the reproductive system (Fig. F) 
the right tentacle is retracted between the 
branches. The penis sac is clavate, containing 
a conical papilla, and produced past the 
retractor muscle into a moderately thick epi- 
phallus half its length ; a very small flagellum 
springs from the latter near the entry of the 
vast deferens. The prostate at its junction 
with the vas deferens is twisted round the 
