4, University of Michigan 



where it is considerably thickened. Through this can be seen 

 ("b") the tentacles and very clearly ("c") the jet-black eyes 

 at the inner base of the tentacles. The pigmented oval spot 

 midway between the base and tip of the tentacles seen in every 

 specimen of Camptoceras sent to me by you (fig. 4) is entirely 

 lacking in this lot (received directly from Mr. Hirase), lead- 

 ing me to believe it is a character not acquired until the animal 

 is nearly or quite mature.* "d" is the head, divided in two 

 large, overhanging lobes, "'e" is the foot. At "f" is a very 

 puzzling character, which I at first thought, from its position, 

 to be a part of the nervous system. It appears to be a band of 

 membraneous tissue, stained with black, deeply imbedded in 

 the animal, "g"' is the jaw, an extremely minute, fragile, 

 chitinous band, of which I have attempted to make an en- 

 larged sketch (fig. 7). Its position is far posterior to w^here 

 one would expect it to be. I have managed to dissect and 

 mount the tube containing this jaw and radula in position and 

 enclose a camera lucida sketch of the result in an endeavor to 

 show the relative position of the two (fig. 8). The plate 

 (fig- 7> g) is dorsal and much stronger with a noticeably 

 curved cutting edge. The two accessory plates (fig. 7, g') are 

 ventral and, while nearly as large, are lighter colored and 

 transparent. The containing wall (fig. 8, j) is distorted and 

 ruptured by the slight pressure of the cover glass at the 

 posterior end and in the region of the radula "I." '"h" is the 

 bent over, anterior end of the radula. Fig. 7 is an enlarged 

 sketch of the jaw. I don't know whether the accessory plates 

 are really attached to the dorsal plate or not. 



* In a previous letter referring to fig. 4, he says: — "They (the eyes) are at 

 the inner base of the tentacles. The black pigmented spot on the tentacles 

 occurred in all four of the specimens I dissected. It is a fact that the eyes and 

 tentacles and large, oval, pigmented spots on the tentacles are exactly the same 

 in Ancylus as in Camptoceras." 



