ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 297 



iron, and phosphoric acid, which, together perhaps with an additional 

 organic substance, form that outer covering so long known as the enamel 

 layer, but hitherto unexplained. The Chitonidae are peculiar in having 

 ferric oxide as the most important mineral constituent of their dark- 

 coloured teeth. 



In the Docoglossa the mineral matter may be 27 p.c. of the whole 

 ribbon, as in Patella vuhjata. In the others it may contribute only 2 • 4 p.c. 

 as in Helix aspersa, though in this species it sometimes rises to 3 ' '^ p.c. 



It seems most probable that the odontoblasts are replaced by fresh 

 cells derived from the cell aggregate at the extremity of the radular sac. 

 Starting from the indifferent cell mass from which they arise by cell 

 division, the odontoblasts become elongated and form a set of cells 

 which possess as a whole a definite and constant shape. They secrete 

 chitin first for the teeth, next for the basal membrane, and are then 

 described as exhausted. But they pass on and become the youngest 

 cells of the basal epithelium. They then travel forwards, adhering to 

 the basal membrane, and become gradually shorter. As they continue 

 their course they encounter the superior tensor muscle, and they have to 

 adhere to the radular membrane on the one hand, and make connection 

 with the tensor muscle on the other. They become liberated from the 

 muscle again and pass forward, now as a low epithelium, until they en- 

 counter the inferior tensor, with which some become connected. After 

 this they once more move forwards to form part of the walls of the 

 sublingual groove which is the natural outcome of the mode of growth 

 of the radula, and which allows of the free play of the buccal cartilages 

 in eating. 



Reproduction of Arion empiricorum.* — H. Lams communicates 

 some notes on the sexual functions in Arion. The animals are protan- 

 drous, ovarian development occurring from September to November, 

 Fertilisation is reciprocal, and takes place at the end of the male period 

 while the eggs are immature ; it is often intra-ovarian, and eggs may be 

 found in the morula and blastula stages in the interior of the ovary, 

 although this is not usual. 



5. Iiamellibranchiata. 



Glandular Organ in Pinna. j — Mario Stenta gives a full account of 

 a pre-oral tubular gland in Pinna, which he calls Poll's gland. It is 

 probably excretory in function, and opens by a lateral duct into the 

 infra-branchial cavity. It produces a fluid secretion, and may take the 

 place of the pericardial gland which is absent in Pinna. 



Fluorine in Shells of Bivalves.^ — -P. Carles has shown the presence 

 of fluorine (obtained from terrestrial fluorides in the sea-water) in small 

 percentage (0'012) in the shells of oysters and mussels. 



Radiography applied to Discovery of Pearls. § — Pt. Dubois points 

 out that in I'JOl he submitted radiographs of a pearl inaide 3Iargarita7ia 



* C.R. Soc. Biol. Paris, Ixii. (1907) pp. 255-7. 



t Arbeit. Zool. Inst. Univ. Wien, xvi. (1906) pp. 407-36 (1 pi. and 1 fig.). 



t Comptes Rendus, cxliv. (1907) pp. 437-8. 



§ C.R. Soc. Biol. Paris, Ixii. (1907) pp. 54-5 (1 fig.). 



June 19th. 1907 X 



