212 Monograph of the Lamellibranchiata. 



able, however, that these equivalve shells, with a large byssus, belong 

 to neither of these families, but to some other family. The fact is, 

 that, until quite recently, but little attention was paid to the internal 

 structure of Lower Silurian shells belonging to this class, and conse- 

 quently but little was known of their fami^ly relations. The hinge 

 lines of the Avicnla, Amhonycliia, CijpricanUtes and Tellinomya, have 

 been known for only a few years; the hinge line of Anodontopsis, if it 

 be the hinge line of that genus, was first figured in the Ohio Pale- 

 ontology, in 1873; and within the present year I have figured and 

 described,, for the first time, the hinge lines of the Anomalodonta, Mo- 

 diolopsis and Cijcloconcha ; while we are yet wholly ignorant of the 

 hinge lines and internal structure of the Oriho)iota, Cleidophorm, and 

 other genera, and numerous species, belonging to tliis class, and found 

 within the Cincinnati Group. 



FoT these reasons [and the further fact that I have not had the time 

 to devote to the study of the generic affinities of these shells, necessary 

 . to trace out their relations, or to throw any particular light on the sub- 

 ject], I have omitted the arrangement of the genera into families, in 

 this monograph. It is a matter of very grave doubt whether any per- 

 son is justified in describing a species, when he finds it necessary to 

 follow the generic name with an interrogation point, and especially 

 from a mere cast, and there can be no doubt that he is wholly inex- 

 cusable for attempting to arrange genera into families without full con- » 

 fidence in the correctness of the classification. 



I have described one new genus (Cycloconcha) , and two new species, 

 for this monograph, and have omitted to include any species about the 

 existence of which, in the Cincinnati Group, I entei-tain any doubt. 

 It is not a diflicult thing to find casts that we are unable to specifically 

 identify, but to give each of them a name, or to call them by the 

 names of the casts that have been figured from New York, Canada, or 

 elsewhere, from a slight resemblance merely to the engravings, would 

 not add anything to our knowledge of the science, but would rather 

 be a stumbling block in the way of advancement, consequently, I have 

 omitted them. It is an indisputable fact, however, that several new 

 species will be added to this list, either by new descriptions or by 

 future identification with those already described elsewhere. 



Class Laviellibranchiata — (Bl AIN ville) . 



This class has two shelly valves, placed at right angles to their 

 position in the Palliobranchiata, one valve being, applied to each side ; 



