BIVALVIA. 99 



views more precise and conclusive. The considerable opportunities afforded us for in- 

 vestigation, and the interest with which we have long viewed this obscure family, combine 

 to impart to our language a degree of confidence which we should not otherwise venture 

 to express. The numerous and varied series of these fossil forms all agree in having their 

 test of great tenuity and delicacy, so that not unfrequently we are reduced to derive our 

 knowledge from an examination of their internal casts ; or, should the tests be preserved, 

 it is very rarely that we are enabled to expose sufficiently their hinges or other internal 

 characters. In this family we also lose another important aid in the determination of the 

 genera, inasmuch as the dental characters of the hinge are reduced almost to nothing, 

 the Oolitic Myadas being altogether destitute of hinge teeth, properly so called, and 

 possessing only a shelly lamina, variously modified in form, and extending internally 

 posteriorly to the umbones, and supporting the cushion of the ligament ; but this lamina 

 never forms an elevated nymphal collosity, as in certain recent genera. 



At the period of the publication of the ' Etudes Critiques,' the internal hinge characters 

 of certain of the genera had not been fully ascertained. They were known only from 

 appearances upon the external moulds or internal casts ; and in more than one instance the 

 author was induced to rely upon the observations of others, although these were opposed 

 to his own experience. These uncertainties have since gradually been diminished, not, 

 indeed, without the perpetration of other errors, and it will be found that in the presen t 

 Monograph, we have been induced to adopt certain modifications of, and other changes 

 in, several of the genera, although our exemplifications of the Myadae constitute only a 

 subordinate position in the testacea of the Great Oolite. 



In discriminating the fossil Myadae, it will be found that certain features, which are 

 only of subordinate importance in shells of the symmetrical acephala, generally become the 

 principal, and, indeed, sole aids upon which we have to rely ; fortunately, however, these 

 features, which are included in the terms general figure and ornaments of the surface, 

 acquire in the Myadas an increased degree of importance from their invariable persistence 

 and distinctness of design, in a similar ratio that the hinges and their characters have 

 degenerated in value. 



The thin flexible coverings of the fossil Myadas have a much more intimate relation to 

 the forms of the enclosed Mollusks than is possessed by the shells of other families of 

 bivalves ; the shell does not form a mere compact rigid cyst, but rather a thin sheath or 

 tegument, which conforms to the figure of the Mollusk itself, and varies somewhat 

 according to the circumstances in which the animal was placed with relation to the 

 surrounding ground, or to contiguous organisms. The entire family have large, irregular, 

 longitudinal folds or ridges, which are, for the most part, but imperfectly distinguishable 

 upon the internal casts. The genera of Myadae, proposed by Agassiz, are the following. 

 Pholadomya, Homomya, Corimya, Ceromya, Cercomya, Goniomva, Myopsis, Pleuromya, 

 Arcomya, Platymya, and Mactromya, Pholadomya had previously been established, and 

 remains uncontroverted. 



