348 



KOBERT TRACY JACKSON ON THE 



A similar hinge plate exists in Pecten monolimeris, Cow., and it is described by Dall (IG) 

 in the young- or adult of several species of Pecten, and in Pseudamusiuni and Diuiya. 

 Tliis fluted plate, Dall considers as a form of dentition augmenting the streugtli of tlie 

 hinge line (on the hinge of Pelecypods, etc., Am. Jour. Sc. Dec. 1880). 



Tile right valve of Pecten magellmucui^ has a thin layer of Avell defined piismatic 

 structure, tig. 39, and the prisms are lai-ger than those observed in other s])eeies of Pec- 

 ten. They are peculiai- in being for the most part almost quadrangular, and are arranged 

 with extreme regularity parallel to the lines of growth; but where slight fractures have 

 induced irregular shell growth, they are polygonal as in other species of Pecten. The 

 right valve in the young specimens studied seldom shows radiating stri;e but they some- 

 times exist slightly emphasized in the last-formed poition of the shell, as series of dis- 



Fio. 40. coiuiecttHl knobs. The ventral ear of the right valve, 

 on the contrary, beai's coarse costai crossed by imbrica- 

 ting lines of growth. In the adult, the radiating ridges 

 of the right valve are much (latter and less accentuated 

 than in the left upper valve. The left valve, fig. 38, 

 after the earliest nepionic stages, whic-h are smooth, is 

 inarked by a stage of concenfric and i-adia(ing I'idges 

 both shar[)ly defined and dividing the sliell surface into 

 sunken squares. Knobs or bosses ornament the inter- 

 section of the I'idges. The structure seen hei-e is a com- 

 mon form of surface ornamentation among fossil Avi- 

 culidje. This peculiar marking of the early stage is 

 very characteristic althougli limited in extent and ceases 

 plate byssai sinus, suuiiim-kin;; of the U'ft Suddenly being succcedcd by line somewhat sinuous 



viUvo cUariu-ti'vistic of tXw. period of ile- 

 velopnuMit. X IS diam. 



Fic. ;il). — I'eclen ituKjcllanicns, prismatic 

 slriietiire of rijiht valve. X 'JO diam. 



Fig. -10. — I'ectcji dislocatua, prismatic 

 structure of rijrlit valve. X '.»0 diam. 



Orif^inal speciuions in Sinithsoniau In- 

 stitution, AVasliington (drawn by J. II. 

 Kmerton). 



FIO. 39, 



Fig. 38. — Left and right valve of yonng 

 Pecten inariBllnnicns. showing lUitcd liinge 



radiating ridges which are retained thereafter, and 

 are chai-acteristic of the adult. 



Some adult specimens of Pecten nuujellaiuciis, from 

 the Georges' Banks, are interesting for a comparison 

 with the observations on anatomical features of P. irra- 

 dians. The species is well fitted for study on account 

 of the size, adult specimens measuring \~) cm. or more 

 in lieigbt. The adducloi- muscle is double and the two portions are strongly accent- 

 iiated. The borilers of (he two poilions of (he muscle are free on their limits and at 

 the point of insertion in (iu; shell ])roduce (wo wt'll-inarked impressions instead of a 

 single sligh( one as in Pecten irnidiaus. '^fhere is no pos(erior re(rac(or muscle of the 

 foot in (his species; but it quite likely exists in the young, as that bears evidence in its 

 shell, fig. 38, of being an active crawler. The foot is marked by two clefts, the distal 

 one for crawling, the proximal one the seat of the liyssal gland as in Pecten irradlaiis. 

 The mantle as in that species, has a thick marginal perpendicular wall, decreasing at the 

 eared areas and thickly set with guard tentacles, extensible tentacles and eyes, indicat- 

 ing similar hal)its to those described in Pecten irradians. The gills, intestine, and re- 

 produc(i\e body are also (he same as may be observed in Pecten irrcuKaii>i. 



Chlanijjs (Pecten) iskmdica, Chemn. s[)., s|)eeimen No. 272i)l, Smidisonian Institution 



