242 NATURAL SCIENCE. April. 



It is a minor point that section I. comprises the Solemyidae alone, 

 which are thus made equivalent in importance to the whole of the 

 rest put together, whereas theit anatomical affinities are nearest those 

 of the Nuculidae, whence it would, perhaps, have been better to have 

 let them rank as a group of equivalent value to those in section II., 

 under some such name as " Lipodonta." 



Coming to the next group, Taxodonta, we find two " super- 

 families " (by which term our author appears to indicate something 

 slightly less in importance than a suborder) linked together, yet 

 differing so considerably, not only in their anatomy, but also in their 

 hinge-characters, that the fact of both possessing a great number of 

 teeth in a row seems, in the light of modern biological research, 

 hardly sufficient justification. 



A more startling surprise, though, is the association of such 

 diverse forms as are here assembled under the section " Schizodonta." 

 Similarity of hinge-characters there certainly is none, nor, perhaps 

 v/isely, has a definition of the section been vouchsafed. Ostvea, 

 when having traces of dentition, is Taxodont ; in the Unionidae 

 alone are three types of hinge. Nor is a bond to be found in the 

 gill-structure, the Trigoniacea being Filibranchia, while in the 

 Ostreacea and_^Pteriacea the gill is more or less reticulate, and in 

 the Naiadacea exhibits a very complicated structure. In Pteriacea 

 and Ostreacea the ventricle is ventral to the rectum, in the others 

 it is traversed by the latter. In short, it is difficult to see what 

 connecting link exists between them. The sole reason that seems 

 to have actuated Dr. Dall would appear to be that Philohrya, which 

 is "probably allied to Avicula . . . passes through a Glochidium stage, 

 thus adding another link to those which connect the Aviculidae with 

 the Naiades," and that where the Unionidae go Trigoniae must follow. 

 Among the Naiadacea it is to be noted that Dall tentatively places 

 the Megalodontidae, induced, seemingly, by their resemblance to 

 some forms of North American Unio. The statement concerning 

 Pliilohrya, which doubtless will be confirmed by future observation, is 

 extremely interesting but not conclusive. Parasitism in the young 

 stage does not necessarily imply affinity, any more than it does in the 

 adult. 



Concerning the Isodonta, despite those who lay stress on the 

 prodissoconch, it may be observed that Pectinacea and Anomacea do 

 not, on anatomical grounds, seem happily yoked together. It is 

 worthy of remark that in the young state the byssus of Anomia 

 issues from the ventral margin of the shell, as in Ayca. 



Dr. Dall points out, and we believe the observation is new, that 

 in its juvenile stages Spondylus has a Taxodont hinge; but he does not, 

 as in the similar case of Chamacea, propose its removal from its allies 

 on that account. 



The Dysodonta, containing the well-defined Mytilacea, call for no 

 remark beyond the expression of belief that the restitution to them of 



