240 PROCEEDINGS OF THE ACADEMY OP [1893. 



but isolated in this state. In tbe first lateral tbe mesodont is large, 

 as long as tbe plate or more so ; then it becomes gradually shorter 

 till in the last perfect lateral it scarcely reaches beyond the middle 

 of the plate. The ectodont is always small, but in some instances I 

 have seen it longer in the last lateral than in the one preceding. 

 W. G. Binney says (L c.) that the posterior mesial (inner) angle of 

 the plate is suppressed. As a rule, I could see that angle well 

 formed in isolated teeth, althougb not projecting. 



Outside of tbe laterals follow 2 or 3 transition teeth, of which at 

 least the first mesial might range with the laterals, as there is a 

 well formed though somewhat shorter plate with the distal angle 

 not projecting and two cusps ; the mesodont becomes longer 

 again and the ectodont simple or subsimple, but finely double- 

 pointed, which splitting becomes more marked in tbe next follow- 

 ing tooth, thus resembling more the true marginals (PI. YIII, A, 5 

 and 6, B 5, C 5 and 6, D 7, E 6, F 6.) 



The mesodont is of a peculiar formation in these transition teeth. 

 As illustrated in PL VIII, A 4, B 5, C 5,D 5 and 6, F 4 and 5, it 

 becomes obvious that the true mesodont is rather disappearing, 

 while, in fact, a different cusp, another element, grows out from it, 

 or ratber, on tbe mesial side of it, in close connection with tbe plate 

 of attachment, which then becomes the mesial cusp of the true mar- 

 ginals, in which the plate of attachment does not reach beyond tbe 

 reflection. In these, the distal cusps, corresponding to tbe multi- 

 plied ectodont, increase in number up to 3-5 and even 6, gradually 

 becoming smaller and obsolete toward tbe distal end of the single 

 tooth as well as of the row ; so that, as a rule, the last tooth is 

 simply a small, wide, transverse bar, without any perceptible cusps. 

 At the same time these cusps become longer, like the teeth of a 

 comb, so that tbe marginals may be said to be rather pectinate than 

 serrate. In some species, there is a peculiar formation of the middle 

 marginals, especially in V. excentrica (PI. VIII, fig. B, 7-1 0), in 

 which the cusps do not stand singly on the base, but on a common 

 socle, thus giving the tooth a fan-like appearance. The same is 

 seen, but in a lesser degree, in V. cyclophorella, PI. VIII, fig. E, 7- 

 11. In these teeth the mesial cusps are particularly directed in- 

 ward. 



Shell. — It has already been said that the shells of all the Vallonia 

 species are remarkably uniform — there is moderate diversity in size, 

 the smallest averaging 2 - (some specimens P7) the largest about 



