replaced ti^r Purpura patula pansa Gould. It is interesting to note that the size of Purpura 

 found on the Pearl Islands varied from U to 5' 5 cm., while rmich larger specimens reaching 

 7 cm. in length were found along the mainland coast. 



The middle portion of the tidal range is occupied "by ver;.' large nuialDer of small 

 oysters, Ostrea cumingiana mexicana Sowerby. The species is distributed from Lov/er 

 California to Central America. All of the specimens in my collection were small, rarely 

 exceeding 2.5 cm. in length. The interior of the shells is olive green, while the exterior 

 is white. The margins have many small plications by v/hich the species can be easily 

 distinguished from other oysters found in Panamanian v/aters. The oyster sets in great 

 abundance on rocks. A broad v/hite band from 2 1/2 to ^■ feet wide, consisting almost ex- 

 clusively of shells of this species, is very conspicuous on the dark brown background of 

 tidal cliffs, and is noticeable almost every\/here along the shores of the Pearl Islands 

 (Pig. ih) , Since the rocks above and below the setting zone do not appear to be different 

 from those occupied by the oysters, an explanation for a great intensity of setting in a 

 rather narrov; band within the tidal range must be looked for in the behavior of the oyster 

 larvae, vAich for some reason must concentrate at this particular level. At the time of my 

 observations, the gonads of this oyster were ripe, but no larvae were found in the Tjlankton. 



Approximately at the level occupied by 0. cumingiana mexicana , and below it, one 

 freo_uently encounters large Chiton stockesil Broderip, usually occupj^ing small holes in the 

 crevices of the rocks, many compound ascidiae, and Pseudochama ectoQrra (Conrad). The shells 

 of the latter lamellibranch v;ere so overgrown by small barnacles and other fouling organ- 

 isms that they were almost undistinguishable. Small crevices, ledges, and depressions in 

 rocks abounded in various gastropods, apong v/hich the more numerous were Turbo saxosus Wood, 

 Cs-lliostoma iridium Dall, Leucozonla cingulata Lamark, Crucibulum imbricatum Sovjerby, 

 Astraea buschii Philippi, and brilliantly colored Pissurelidae. Less abundant were 

 Phasiaaella sp. , Tegula-sp. , and various small cones. 



Pigure lU Young oysters, Ostrea mexicanfl. , on rocks about the 



middle of the tidal range. Pearl Islands. 



25 



