772 



SCIENCE. 



[N. S. Vol. IV. No. 100. 



by Bernard is that the so-called '■ cardinal ' 

 teeth in the Teleodesmacea are genetically- 

 identical with the distal laminae called in 

 the adult shell ' lateral ' teeth. The ante- 

 rior primitive lamellae grow, and, as they 

 grow, curve and develop angular hooks at 

 their proximal extremity. These hooks 

 become detached from the main body of 

 the lamella from which they originate and 

 the distal part of which become ' lateral,' 

 while the hook develops into a ' cardinal ' 

 tooth. In such a form as Bangia the hook 

 remains permanently attached to the lower 

 anterior lateral by cessation of development. 

 The disunited portions of the hook may re- 

 main separate or, as in Mactra, unite with 

 one another and so form /\ -shaped teeth. 



In general, the results of Bernard's work 

 appear to confirm the unity of the groups 

 of Prionodesmacea and Teleodesmacea, as 

 formed by the writer, and to approximate 

 to the latter the group of Anomalodesmacea, 

 as already intimated by me. He has also 

 in some details eificiently reinforced the 

 dynamical doctrine as explanatory of many 

 features in the growth and resulting form 

 of the shell. The invalidity of N"eumayr's 

 Desmodonta, already fairly proven by Bitt- 

 ner and the writer, is confirmed, while the 

 moribund order, Septibranchia, finds no sup- 

 port in the development of the hinge. 



It has long been known that the gill of 

 Planorbis is a flat lobe, not lamellose like 

 most external molluscan breathing organs, 

 and recently Pelseneer * has reviewed and 

 added to our knowledge of this organ and 

 analogous structures in several gastropod 

 types. This gill plate is not homologous 

 with the typical prosobranch ctenidium, 

 but is an independent development acces- 

 sory to the lung of the fresh water pulmo- 

 nates. It occurs in one form or another in 

 most of them, even Limncea showing a rudi- 

 ment in some cases. In our Ameria scalaris 

 Jay, from Florida, the plate is large and 



* Arch, de Biologie, XIV., pp. 357-393, 1895. 



smooth, as in Planorbis. In one of the 

 Physiform planorboids from the southern 

 hemisphere, which at one time were gen- 

 erally confounded with the true Physidce 

 in default of a knowledge of their anatomy, 

 Pelseneer describes a further step in the 

 evolution of this organ. In the Ameria 

 lamellata Smith, of Madagascar, the plate is 

 transversely folded into lamellae, as in the 

 great majority of molluscan gills, thus giv- 

 ing another example of the ease with which 

 similar but non-homologous breathing 

 organs are developed among moUusks, a 

 feature which I have long insisted on. ISTo 

 cases of this kind have been known among 

 Pelecypods hitherto, but recently Bernard * 

 in an interesting paper on a new commensal 

 bivalve mollusk, Scioberetia australis, has d'C- 

 scribed a case where the reticulated true 

 ctenidium is formed by the folding and sub- 

 sequent perforation of a single plate, in- 

 stead of the growth and subsequent concres- 

 cence of single filaments. The latter pro- 

 cess has been supposed to be almost, if not 

 quite, universal in the Teleodesmacea and 

 Anisomyarian Prionodesmacea. Bernard sug- 

 gests that the mode of development in some 

 of these may really be similar to that of 

 Scioberetia, and shows that the whole sub- 

 ject requires further stud3^ 



More recently still, Simroth,f in a valu- 

 able memoir on the Pelecypoda of the 

 pelagic region, has described a new type, 

 Planhtomya Henseni, a minute and seemingly 

 strictly pelagic bivalve, in which the true 

 ctenidium has the simple lamellar form of 

 the gill of Planorbis, without transverse 

 plications or free filaments. So the evi- 

 dence grows of how structurally similar 

 breathing organs, whether strictly homolo- 

 gous or not, may be developed either as a 

 orm of the true ctenidia, or elsewhere, in 



* Bull. Scientifique de la France, XXVIl., pp. 362- 

 395, 1896. 



t Die Acephalen der Plankton Expedition, Leipzig, 

 1896. 



