PHYLOGENY OF THE PELECYPODA. 351 



valves. The spines of the left (free) valve are comparatively sharp and aeicular, while 

 those of the right (attached) valve are flatter and spathulate ; though both forms of spines 

 may be serially traced to the imbrications of the young, where they are alike in both valves. 



Spondylus as a genus of attached Pelecypoda naturally draws attention in these studies. 

 It is cemented by one valve and its irregular, inequivalvular shell, PI. xxvi, fig. 1, has 

 been ascribed to the conditions of cemented fixation (vide pp. 318 and 323) . The cam- 

 erated structure of Spondylus has been also considered and I wish here only to describe 

 the young. 



In the collections of the Boston Society of N'atui-al History are specimens of Spondijlus 

 asjJen-i/mus, Sby., from the Pacific Islands, bearing the catalogue luunber HIOI. The up- 

 per left valves of the specimens are j^roduced on the hinge line as slightly developed ears 

 and have a decidedly pecteniform aspect, as is characteristic of this valve in living and 

 fossil Spondyli. The lower right valves which are or have been attached, are composed 

 of highly irregular, concentrically rugose and spinous shell growth, as is also characteris- 

 tic of species of this genus. One of the specimens of a right valve, however, is very well 

 preserved up to the initial stages of growth. At the umbo of this well-preserved speci- 

 men of Sj}Otidylus asjjerrimus, PI. xxvrr, fig. 4, is a prodissoconch similar to that described 

 in species of Pecten. The prodissoconch is rounded or oval in outline and is clearly marked 

 olF from the succeeding shell growth.' It is not sufficiently well-preserved to show lines 

 of growth ; but it has no byssal notch which exists, however, in the first stages of the 

 succeeding dissoconch growth, as in Pecten, PI. xxvii, fig. 9. The first nepionic stage of 

 shell growth in Spondylus, as shown in the figure, is pecteniform. It has a long hinge line 

 relatively to the width of the shell at that age, and a deep byssal sinus. As in the pro- 

 dissoconch most of the lines of growth of this early stage have been worn ofi". At the 

 close of the pecteniform stage the animal became soldered to a foreign body and the suc- 

 ceeding shell deposition, aftected by the new condition of cemented fixation, has assumed 

 the irregular and rugosely distorted growth chai-acteristic of young Spondyli. In the 

 first stages of the irregular growth, the byssal notch is soldered over, and eradicated in a 

 similar fashion to that shown in Hinnites, PI. xxvi, fig. 3. A prodissoconch and nepi- 

 onic stage similar to that described in Sjwndylus asperrimus, has been observed also in 

 specimens of S.croceus, Chemn. and 8. variegatus, Chemn. sp.?, at the N'ew York State 

 musemn; and in S. dacalis, Chemn. and S. varians, Sow., at the Peabody Academy of 

 Science, in Salem. 



The above description of the development of the shell of Spondylus is directly com- 

 parable to what has been described in the shell of Hinnites. Both valves before fixa- 

 tion are pecteniform in these genera; as soon as attached the lower valve loses this early 

 existent, ancestral feature and assumes the irregiUai-ity characteristic of attached Pele- 

 cypoda with of course the peculiar order of irregularity normal to Spondylus or to 

 Hinnites. As the lower valve becomes iri-egular and distorted, the upper valve in both 

 genera, acting in sympathy, becomes irregular also; but, as previously claimed, the dis- 



'A specimen of a left valve of a young Spondylus, in tlie valve. It is rounded, with slightly developed uml)os. Its 



Smithsonian Institution collections, catalogue No. ()2242, limits are mai-ked oil' from the succeeding dissoconch as in 



shows a prodissoconch like that described above in a riglit PI. xxvn, flg. 4, of a right valve of S. asperrimus. 



