HSrVERTEBRATK PALAEONTOLOGY. 439 



fully described by the writer in 1865, on pages L18 d> 1"21 of a memoir 

 published by the Smithsonian Institution on the Palaeontology of (he Upper 

 Missouri, in the joint names of Meek and Hay den, already cited in connec- 

 tion with the genus Scapkites. From that description and remarks, the fol- 

 lowing paragraphs are quoted, with some modifications, and references to the 

 figures on the plate here given, instead of to the wood-cuts inserted in the 

 text as originally published. 



" The specimen here under consideration is thin, and seems to have been 

 composed of a single lamina showing no cellular structure, and possibly con- 

 sisting only of the inner layer. It is the only organic body found in the frag- 

 ment of the shell : the surrounding space being filled with fine, indurated, 

 sedimentary matter, similar to that in which the shell was originally envel- 

 oped. It occupies a position apparently about one-fourth of the distance 

 back from the aperture to the first septum, and lies with the two valves 

 partly open and in their natural position with relation to each other; their 

 straight edges being joined together, and deflected so as to form a distinct 

 carina, come in contact with the peripheral side of the shell, as shown in 

 figure 3, d, plate 35; while the truncated or left ends of the two valves, 

 as they would appear if opened and flattened out, as seen in figure 3, i, are 

 directed toward the aperture of the shell. 



"The fact, however, to which especial attention is called, is the occur- 

 rence of a third piece, or appendage (see figures •">, A g. h, representing an 

 under, lateral, and upper view of this part as seen in figure 3, d, in place, 

 exposed by the breaking away of a part of one of the valves), differing 

 entirely from either of the two valves already noticed, and, so far as known, 

 from anything previously found in connection with any of these fossils. This 

 third piece, as already explained, occupies a position between the two valves 

 as they lie together partly opened, being nearer the extremity directed 

 toward the aperture of the shell, and exactly fitting between the valves, as 

 if in its natural position with relation to them. It agrees so exactly in thin 

 ness, texture, and surface-markings, and even in color, with the two valves 

 enveloping it, that it is impossible to examine the specimen for a moment 

 and entertain a doubt in regard to the three pieces being all parts of the 

 same fossil.* 



* It has been maintained by some tbat an Aptycbus, as usually understood, has its two parts not 

 merely joined together along the straight edges, but actually anchylosed there, so as really to consist of 

 one bilobate piece. If this is bo. of course what' il. here called a third piece, would really be only a 



-Ml crllfl f>l»-(e. 



