INSTITUTE OF SCIENCE, PHILADELPHIA. 437 



a great many recent Dciitalia, and have never seen a specimen in which the 

 " tube-in-tube " was not obviously the result of the above process, and I 

 believe it always to be so. Such forms as D. bihibatnni Meyer, from the Jack- 

 sonian Eocene, and several recent forms illustrate the conditions mentioned. 

 D. tarcntinuin is especially liable to such breakages and repairs. It will be 

 understood that it is not asserted that, from a peculiar fragility or liability to 

 transverse breakage in a species, this condition may not be almost habitual 

 with the adults of that species ; but it is undeniable that no one has ever 

 recorded a specimen with the posterior end entirely unbroken and yet possess- 

 ing the supplementary tubule. 



Another form of repair is sometimes observed in species which normally 

 have a dorsal wave or sulcus in the posterior orifice. Here not only will the 

 broken tip be, as it were, double-lipped, but a slight absorption will take place 

 in the middle line above, corresponding to the sulcus, even in the solid shell 

 of the truncation. Such a state of affairs has been figured by Meyer (Bull. 

 Ala. Geol. Survey, I., pi. I, fig. 2 a and pi. 3, fig. 2 a) in specimens of D. Lcai 

 and D. Danai Meyer, but it is never what may properly be called normal, 

 though occasionally it may have become habitual. 



Those who have studied large numbers of Dentalia will have been struck 

 by the extreme sharpness and tenuity of the posterior portion of the young shell, 

 which is almost invariably lost long before maturity has been reached, and will 

 realize that only a carefully graded series connecting the very young with 

 the adult will give anybody the means for describing the normal form of the 

 posterior orifice with exactitude and accuracy. 



Still another pitfall is to be avoided in studying the characters of the 

 posterior part of the shell. As has been stated, the posterior orifice often has 

 a dorsal slit, very narrow and prolonged in some cases. But it often happens 

 that erosion, especially in specimens from deep water, modifies and sometimes 

 simulates such slits, introducing them where they normally should not be or 

 lengthening them abnormally. There seems to be a peculiarity of some kind 

 in the external prismatic layer of Detitalium, which lends itself to the propa- 

 gation of erosion in longitudinal lines very much more effectively than at 

 right angles to such lines. Hence we see specimens of a species, normally 

 provided with a short slit, exhibiting an enormously long slit, or, starting at 

 some little defect of the posterior margin, a narrow line of erosion, simulating 

 a slit, may run a long distance up the shell. These abnormalities may usually 

 be discriminated by comparison with numerous specimens of the same species. 

 In cases where the student has only one or two specimens, he should refrain 

 from putting reliance on characters which may be abnormal as a basis for 

 describing new forms or for discriminating old ones. 



It may also be added that it rarely happens that smooth species do not 

 show at least a little sculpture near the posterior end, or that sculptured ones 

 do not show a modification of the sculpture toward the anterior end. Hence 



