ZOOLOGY AND BOTANY, MICEOSCOPY, ETC, 229 



( = the " porcelain" layer of Eose) made up of spindle-shaped needles 

 of calcifying substance with a very small admixture of organic matter. 

 The iridescence of the "mother-of-pearl" layer is due primarily to 

 the above-mentioned fine lamellar structure, and, secondly, to the 

 optical properties of the organic membranes. Finally, the organic 

 membranes of all layers are in themselves biaxial. 



The author was thus reduced to investigate the question of the 

 heteromorphy of the calcification by subjecting like portions of the 

 several layers, compared with like masses of calcite and aragonite in 

 various modes of aggregation, to the action under atmospheric pressure 

 of water through which for two months was passed a continuous 

 stream of carbonic acid gas. The results proved that the state of 

 aggregation was the measure of degradation. Compact masses were 

 readily dissolved, whereas looser aggregations — alike of calcite, 

 aragonite, or layers of shells — resisted dissolution in a degree com- 

 mensurate with the compactness of their structure. Thus, in shells, 

 the outer, compact, "honeycomb" layer resisted degradation much 

 more than the scaly organization of the "mother-of-pearl" layer, and 

 this latter again proved more resistent than the fibrillar "ivory" 

 layer. This result is in harmony with geological experience, according 

 to which the last-named layer is rarely found, and the compact 

 " honeycomb " layer is the best preserved. 



Hinge of the Shells of Bivalves.* — This has been systematically 

 examined by M. Neumayr, who suggests the following classification 

 of the bivalved MoUusca : — 



I. Cryptodonta (PalseoconchEe). PalaBozoic, thin-shelled forms, 

 especially Silurian, with no teeth to the hinge, or at best 

 feeble traces, and an entire pallial impression. 

 e. g. Dualina, Antipleura, &c. 

 II. Desmodonta. Teeth absent or irregular, developed when 

 present in intimate relation with the ligament-bearing 

 portions of the shell. 



e. g. PJiolas, My a, Mactra, &c. 



III. Taxodonta. Numerous teeth, undifferentiated, arranged in 



a series. 



e. g. Area, Nucula. 



IV. Heterodonta. Teeth few, clearly differentiated into alter- 



nating cardinals and laterals, which fill corresponding pits 

 in the opposite valve. 



e. g. Cardium, Astarte, Cyprina, Donax, &c. 

 (All the above four groups have two similar adductor-muscle 

 impressions, and together form the old class of Homomtaria.) 



V. Dtsodonta (Anisomyaria). Teeth absent or irregular, with 

 either two very dissimilar muscular impressions, or only 

 one. 



(a) Heteromyaria. 

 (yS) Monomyaria. 



* SB. K. Akad. Wiss. Wien, Ixxxviii. (1884) pp. 385-420 (1 pi.). 



