INSTITUTE OF SCIENCE, PHILADELPHIA. 5O9 



was first described as a parasite under the name o{ glocliidinm. The only ma- 

 rine form yet shown to possess a glochidium stage is the Aviculoid genus 

 Pliilobrya Carpenter. It is not found in the South American AT/z/^Z/cfe, which 

 have a different type of larva, named by Ihering a Lasidhnn. 



General Classification of the Pelecypoda.— Having referred to the 



chief features of the pelecypod type which are available for systematic use, 

 and their variations, it remains to adopt for use in the subsequent descriptive 

 portion of this work such an arrangement as shall best express the relations 

 of the groups and at the same time take account of those characters which are 

 available for the Paleontologist. Continued study has confirmed me in the 

 opinion that, in its general features, the classification of 1889 proposed by me, 

 as now revised, comes nearer to meeting the requirements of the case than any 

 other which has fallen under my notice. 



There are several questions which, in the present state of the science, can- 

 not be finally answered, and any decision upon them must be subject to differ- 

 ence of opinion. One of these is the disposition to be made of the synthetic 

 types classed by Neumayr under the name o'i Paleoconclia. These forms with- 

 out doubt represent more or less clearly the earlier forms, now unknown, from 

 which many of the modern genera have descended. Some of them are off- 

 shoots which came to an end without recognizable descendants. Others com- 

 bine archaic features with peculiar modifications which the action of natural 

 selection has since eliminated. Few if any of them are sufficiently salient in 

 their characters to permit of being confidently referred to one suborder of the 

 recent fauna rather than another. 



Some of them exhibit features which recall characteristics of the Taxo- 

 dont forms which were at the same Geological age well established in numer- 

 ous representatives. Others might be taken as the prototypes of Anomalodes- 

 macean or Teleodont genera. Logically all our modern types should converge 

 as we recede in Geological time, and that they do is obvious. Shall we then 

 struggle more or less ineffectually to allot each Paleoconch to its place in the 

 then undeveloped larger groups of later faunas? Or shall we accept the name 

 offered by Neumayr as a temporary resting place for these undifferentiated 

 ancient types ? As a matter of convenience, solely, I am inclined to the latter 

 course, until more is known. 



Among the Paleoconchs Neumayr was disposed to place the recent genus 

 Soleinya Lamarck (more correctly Solenomyd), which by Fischer was placed 

 near the Anatinidce, in the Anomalodesmacea. For some time I was inclined 

 to adopt this view, rather because Solemya oiicved such a unique individuality, 

 apart from its purely archaic features, that there did not seem to be any very 

 satisfactory grounds for a different course. But I have finally concluded to 

 regard the genus as a very aberrant Prionodont for the following reasons : The 

 gills and foot agree very closely with those of the Homomyarian Prionodonts. 

 These might, however, be merely archaicisms in common, mutable and exter- 



