508 Systematic Paleontology 



contractile membrane capable of secreting shelly matter. This, like the 

 shell, may be simply tubular, sulcate, etc., and when the shell is absolutely 

 perfect the posterior end reflects the form of the membrane which 

 secreted it, and which is known from observations on the recent shells 

 to be capable of repairing damages to the calcareous tube which protects 

 it 



" Another modification of the orifice has given rise to much miscon- 

 ception. Species with very thin shells usually live buried in soft mud 

 which measurably protects them, but others with heavy shells appear to 

 be more versatile ;'at all events, if the small end of the shell is accidentally 

 broken off, the animal can repair it, and in species which have a simply 

 tubular mantle and a thick shell the repairs take the shape of a small tube 

 projecting from the blunt end of the large one, as it is impossible for the 

 mantle to secrete a shell which is as large and thick as the original at the 

 point of truncation. I have examined a great many recent Dentalia, 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 



" Another form of repair is sometimes observed in species which nor- 

 mally 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. 2a, and pi. iii, 

 fig. 2a) in specimens of D. leai 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 



