24 CURRENT LITERATURE. 



in the strict sense, appears before tlie Tertiary, their recognisable ancestry 

 appears in the Upper Cretaceous or Lower Eocene. In the present synopsis 

 Dr. Dall deals with this family in a similar manner to that in Avhich 

 he has already treated other Pelecypod families. Numerous corrections and 

 changes liave been made in the nomenclature, and nineteen new species arc 

 described and figiu'ed. 



Ridewood, W. G. — On the structure of the Gills of the Tianudlibranchia. 

 Phil. Trans. Eoy. Soc, 1903 (ser. B), vol. 10.3, pp. 1 17-:284, Gl figs. 

 in text. 



We have already given a l)ricf outline of this valuable work (see ante, 

 1902, p. 143) ; the complete paper is now before us, and we are able to deal 

 with it in greater detail. 



The author states that in this investigation, 2lo species of these Molluscs, 

 belonging to 11.^ genera, are taken into account, and their gills were studied 

 l)oth by means of serial sections and dissected preparat ions. In almost all cases 

 staining was done by means of borax-carmine, followed by picro-nigrosin. 

 After a brief resume of previous work, and some remarks upon the termi- 

 nology, he passes on to consider the Evolution of the Synaptorhabdic Gill. 

 After reviewing the opinions of previous writers, and comparing them with 

 his own observations. Dr. llidewood is led to conclude that in the presence 

 of the many conHicting descriptions in closely allied forms, the phenomena 

 of gill ontogeny are extremely difhcult to elucidate ; and tiiat, pending some 

 further special research, we may " conclude that the perforation or slitting 

 into filaments of a continuous gill-membrane is an infraction of the rule 

 that ontogeny is a repetition of phylogeny." It is of interest to note, that 

 the remarkable diversity of structure exhibited by the ctenidia is not shared 

 by the labial palps. The plication of the lamellae, to which Hancock, and 

 later Duvernoy, attached considerable importance, is now regarded as of less 

 value than the differentiation of principal filaments, which latter are con- 

 fined to the Pseudolamellibranchia and the Eulamellibranchia. They are 

 not, however, met Avith in all the J'seudolamellibranchs, as Pel- 

 seneer imagined, for in some of the latter the gills are homorhabdic ; in non- 

 plicate gills, principal filaments are also Avanting. The author considers 

 that the plication of the lamellae and the differentiation of principal filaments 

 are not to be looked upon as of more than specific, or at the most, sub- 

 generic value. The apical filaments, cilia, chitinovis skeleton, endothelium, 

 calcified rods, intrafilamentar septa, interlamellar jimctions, and interla- 

 mellar extensions of the septa, are each dealt with in detail. The calcified 

 rods, on account of the confusion that has arisen with regard to them, are 

 deserving of more than passing notice. He mentions that these structures 

 are peculiar to the Unionidae and MuUeria, and some authors have failed 

 to discriminate between the thickened bands of chitin, such as occur in the 

 gills of most Eulamellibranchia, and these calcareous rods, Avhich are embedded 

 in the chitin. They are not continuous rods of uniform width extending 

 the full height of the demibranch, but consist of a succession of short rods, 

 Avhich gradually shade off and terminate in the middle of each interfilamen- 

 tar junction. Dr. Eidewood further states that these rods were discovered 

 by Eengarten in 1853, while the best description of them is that given by 

 Janssens, who, in 1893, showed that they are composed of calcium phos- 

 phate, Avith a little calcium carbonate, and a chitin or conchylin base. 



