TETHYS CERVINA DALL AND SIMPSON 43 



are in general about two-thirds the size of those of Tethys 

 dactylomela. 



The visceral mass nearly fills the body cavity of the animal. 

 It is made up of the esophagus, the three divisions of the stomach, 

 and the intestine, the latter inclosing in its windings the liver 

 and the ovotestis. 



Esophagus and stomach. The esophagus is short and rather 

 thin walled, dilating into the very ample first stomach, or in- 

 gluvies (PI. VI, fig. 31, ingl.), a thin walled sack, densely packed 

 with fragments of algae. The whole alimentary canal is spirally 

 twisted from left to right, clockwise. The ingluvies occupies 

 about one turn of this spiral, the second, or grinding stomach 

 (PL VI, fig. 31, m. st.}, together with the third gastric division 

 completing about one-half of the second turn. The anterior end 

 of the ingluvies dilates rather suddenly from the esophageal tube, 

 the posterior end tapering more gently to the broad band like 

 circular constriction in the canal, marking externally the limits 

 of the thick walled, muscular, second, or grinding stomach (PI. 

 VI, fig. 31, m. st.}. This portion is about 4.0 mm. in length by 

 5.0 mm. in diameter at its anterior end, tapering somewhat poste- 

 riorly. Its inner surface bears a number of strong horny teeth, 

 arranged in five somewhat irregular rows, the anterior ones of 

 which contain the smaller teeth, the succeeding ones increasingly 

 larger, and the last two the largest. The tips of this gastric arma- 

 ture meet in the center of the lumen in the contracted condition, 

 thus making a most effective gastric mill. In general the form of 

 these teeth is the same throughout, being that of a four sided 

 pyramid, the base a rhomb in outline with one of the acute angles 

 directed forward. In the largest teeth of the posterior rows (PI. 

 Ill, fig. 21 ), the crest is either single, rounded and bluntly pointed, 

 or wedge shaped, being prolonged into a transverse ridge. In 

 most cases this ridge shows three distinct summits, separated from 

 each other by shallow depressions, which are continued downward 

 upon the anterior face in two deep grooves, while the posterior 

 face is more uniformly convex. The base frequently presents 

 a transverse median depression upon its ventral surface, cor- 

 responding in position to the region of greatest elevation above 

 (PI. Ill, fig. 21 ). The smaller teeth, found in the two anterior 

 rows, have a single groove upon the anterior face, carried up to 



