PHYLOGENY OF THE PELECYPODA. 351 



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

 those of the right (attached) valve are flatter and spathidate; though hoth 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 \)\). 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^atural History are specimens of Sjwndylus 

 asj)en-irmis, Shy., from the Pacific Islands, bearing the catalogue number 8494. The up- 

 per left valves of the specimens are produced on the hinge line as sHghtly 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 comjjosed 

 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 Sjjoudylus asjJerrimus, PI. xxvir, 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 

 off from the succeeding shell growth.' It is not sufiiciently 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 off". At the 

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

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

 the irregular and rugosely distorted growth characteristic 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 Spondylus asperrirmis, has been observed also in 

 specimens of S.croceiis, Chemn. and S. vai'iegatus, Chemn. sp.?, at the iSTew York State 

 nmseum ; and in S. ducalis, 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 irregulai-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 irregular and distorted, the upper valve in both 

 genera, acting in sj'mpathy, becomes irregular also; but, as previously claimed, the dis- 



'A specimen of a left valve of a yoiina; Spondylus, in the valve. It is ronniled, with slightly flcveloped umbos. Its 



Smithsonian Institution collections, cataloicue No. (;2242, limits are marked otf from the succeeding dissoconch as in 



shows a prodissoconch lilie that descriljcd above in a right PI. xxvu, tig. 4, of a riglit valve of S. asperrimns. 



