8 NATURAL SCIENCE. Jan.. 



Another feature of the Neritidae is the presence of a thick shelly 

 deposit (the callus) just without the mouth, on the surface of the last 

 whorl. This deposit is thickest at the margin next the mouth, 

 and generally at this point forms a ridge or even a thin shelf, or 

 septum, which stretches partly across the mouth, reducing it in 

 size. The full edge of this shelf is often serrate, sometimes coarsely 

 so. As the animal grows and adds to its shell, still winding round 

 in a spiral direction, these various deposits advance with it by the 

 simple process of the addition of fresh material on their outer sides 

 (using the word " outer " in its relation to the mouth and the 

 direction of growth), and the removal of a corresponding amount on 

 their inner sides. 



Having prefaced so much, it is easy to understand the interest of 

 what took place in the most eccentric Eocene member of the family, 

 Velates conoideus, Lamk., as described by Mr. Woodward. In this 

 species the growth of the young shell was perfectly normal, but when 

 it had completed about 4-^ whorls a remarkable change ensued ; it 

 ceased to grow spirally, and increased by the addition of fresh layers 

 of shell over the whole surface of the callus as well as round the 

 margins of its mouth, and gained the additional internal space the 

 animal required by the removal of a corresponding amount of shell on 

 the inner side of the callus. It thus changed both its direction and axis 

 of growth — and like the Irishman, it raised its roof by digging out the 

 floor of its tenement. In a full-grown specimen, therefore, nearly half 

 the shell and the internal septum were carved out of former callus, the 

 layers of which can be seen in sections cutting across the walls and 

 running round on the inner surface. A still more remarkable feature 

 is presented in the disposition of the component plates which, in the 

 callus, are so arranged as to provide, in anticipation, the strongest 

 possible structure for the muscle-carrying septum, ultimately to be 

 sculptured out from it. The presence of silica in the outer layer of 

 the shell is also noteworthy, and it is interesting in this connection to 

 remark that one of the earliest describers of this fossil gastropod, and 

 the author after whom it is sometimes called — C. C. Schmidel — 

 observed, in 1780, that this outer layer did not appear to consist 

 entirely of lime. 



The Classification of Bivalved Shells. 



After the embryologist has made a certain advance in the study 

 of development, and the comparative anatomist has progressed in 

 the determination of homologous parts, the services of the systematic 

 zoologist are required to express the results in orderly sequence. 

 Classifications are thus merely temporary expedients — generalised 

 retrospects, so to speak — and every advance demands some recon- 

 sideration. At the present time Professor Carl Grobben (of Vienna) 

 is of opinion that some reform is necessary in the classification of the 



