i893. NOTES AND COMMENTS. 7 



on etymological grounds, but were used with too great laxity. As now 

 revised, the technical terms are as follows : — einhryonic, bvepliic (= in- 

 fantine or larval), neanic (= adolescent), ephehic (= adult), and gerontic 

 (= senile), the last being subdivided into catabatic (= declining) and 

 hypostyophic (= atavic). These terms are restricted to stages in the 

 growth of an individual. By prefixing the syllable phyl- they may be 

 made to express stages in the history of a race ; while the successive 

 stages in the evolution of a character may be designated by the 

 same terms with the addition of the prefix morpho-. Confusion of 

 terms leads most surel}' to confusion of thought, and if the latter be in 

 any way dispelled by the labours of Messrs. Buckman and Bather, we 

 are bound to be irrateful. 



The Growth of Shells. 



A MOST peculiar, and, as far as is at present known, unique mode 

 of increase in a Gastropod shell, is described by Mr. B. B. 

 Woodward in the last number of the Proc. Zool. Soc. London (1892, 

 pp. 528 — 540, pis. 31, 32). Broadly speaking, the Gastropod shell 

 may be looked upon as a more or less elongate conical tube, at the 

 apex of which is the young shell, while increase takes place by the 

 addition of fresh material at the other, open, extremity — the mouth. 

 This tube is usually spirally coiled in the direction of a screw, the 

 successive whorls touching one another. In those forms where the 

 spire is low and the whorls are close together, the portion of the 

 tube next the previously-formed whorl is sometimes largely, some- 

 times completely, dispensed with, and what was in the first instance 

 the outermost wall of the tube becomes in the course of growth the 

 dividing partition between the last whorl and its predecessor. When 

 the shell is a very thick one, the presence of an equally stout 

 internal partition [paries) between the whorls becomes unnecessary, 

 and even inconvenient; hence it is very frequently to a great extent 

 re-absorbed and reduced in thickness by the animal, as, for example, 

 in Conns. 



The Neritidae advance a step further, and usually remove the 

 greater part of the paries altogether, converting the shell into a 

 single chamber, the only remnant of the dividing wall left being a 

 portion near the mouth, to which one of the retractor muscles 

 is attached. On the other hand, this remaining fragment of 

 paries is usually strengthened, sometimes considerably, by an extra 

 layer of shelly material. In extreme cases this deposit forms an 

 independent projecting shelf [septum), standing out from the paries 

 into the general cavity, between the former and the mouth of the 

 shell. In these instances the septum forms the point of attachment 

 for the retractor muscle, and the remnant of the paries is reduced to 

 insignificant proportions or disappears entirely. The different species 

 of Neyitlna and Nevita exhibit the various stages in the process. 



