348 



ROBERT TRACY JACKSON ON THE 



A similar hinge plate exists in Pecten jnonotimei'is, Con., and it is desci-ibed by Dall (IG) 

 in the young or adult of several species of Pecten, and in Pseiuhvnuisiuni and Diniya. 

 This fluted plate, Dall considers as a form of dentition augmenting the strength of the 

 hinge line (on the hinge of Pelecypods, etc., Am. Jour. So. Dec. 1889). 



The right valve of Pecten magellanicus has a thin layer of well defined prismatic 

 structure, fig. 39, and the prisms are larger than those observed in other species of Pec- 

 ten. They are peculiar 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 striic but they some- 

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



FiG. 40. connected knobs. The ventral ear of the right valve, 

 on the contrary, bears coarse costse crossed by imbrica- 

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

 of the right valve are much flatter and less accentuated 

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

 after the earliest nepionic stages, which are smooth, is 

 marked by a stage of concentric and radiating ridges 

 both sharply defined and dividing the shell surface into 

 sunken squares. Knobs or bosses ornament the inter- 

 section of the ridges. The structure seen here is a com- 

 mon form of surface ornamentation among fossil Avi- 

 culidae. This peculiar marking of the early stage is 

 very characteristic although limited in extent and ceases 

 suddenly being succeeded by fine somewhat sinuous 

 radiating i-idges which are retained thereafter, and 

 are characteristic of the adult. 



Some adult specimens of Pecten magellanicus, fi-om 

 the Georges' Banks, are interesting for a comparison 

 with the observations on anatomical featui'es of P. irra- 

 dians. The species is well fitted for study on account 

 of the size, adult specimens measuring 15 cm. or more 

 The adductor muscle is double and the two portions are sti'ongly accent- 

 The borders of the two poi'tions of the muscle ai'e free on their limits and at 



Fig. 39 



Fig. ; 



Fig. 38. — Loft and right valve of young 

 Pecten magellanicus, showing fluted hinge 

 plate byssal sinus, and marking of the left 

 valve characteristic of the period of de- 

 velopment. X 18 diam. 



Fig. 39. — Pecten magellanicns, prismatic 

 structure of right valve. X 90 diam. 



Fig. 40. — Pecten dislocatu^, prismatic 

 structure of right valve. X 90 diam. 



(Original specimens in Smithsonian In- 

 stitution, Washington (drawn by J- U- 

 Emerton). 



height. 



u\ 



uated. 



the point of insertion in the shell produce two well-marked impressions instead of a 

 single slight one as in Pecten irradians. There is no posterior retractor muscle of the 

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

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

 one for crawling, the proximal one the seat of the byssal gland as in Pecten irradians^ 

 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 habits to those described in Pecten irradians. The gills, intestine, and re- 

 productive body are also the same as may be observed in Pecten irradians. 



Chlamys (Pecten) islandlca, Chemn. sp., specimen No. 27294, Smithsonian Institution 



