126 A COMPARISON OF SOME PECTENS FROM THE EAST 



instance, Pecten latiauratus. The proportions of the dorso-ventral diameter to 

 the transverse half-diameter (height of a single valve) are closely similar in the Tampa 

 and San Diego lots. Thus the ratio of transverse half-diameter to dorso-ventral 

 diameter is as follows: 



Tampa. San Diego. 



Right valve 32 .34 



Left valve 265 .295 



Consequently, applying even the delicate test of proportions, the Tampa and San 

 Diego shells show a close similarity. I conclude, then, that the Tampa and San Diego 

 Pectens are closely related and that, environmental factors being the same, the variability 

 should be the same. Any considerable difference of variability is probably due to a differ- 

 ence in the action of environment. 



IV. RELATION OF THE FOREGOING FACTS TO THE THEORY OF 

 MIOCENE CONNECTION ACROSS THE ISTHMUS OF PANAMA. 



Having demonstrated the close relationship between the Tampa and San Diego 

 Pectens the question arises: How is it possible to have so closely related forms sepa- 

 rated by so great a distance of coast-line? In this dilemma, as in many similar ones, 

 assistance is given by the fact now well ascertained that the Pacific Ocean on the one 

 side and the Caribbean Sea and the Gulf of Mexico on the other were continuous in 

 Miocene times. At this period, no doubt, many of the species of the two sides of 

 Central America were identical. Pecten irradians-gibbus-ventricosus was one of 

 those. Since the uplift at the end of the Miocene most of these common species 

 have become differentiated so as to be distinct. But it has been estimated (see 

 Fischer, '87, p. 169) that three per cent, of the mollusks of the two sides of the Isth- 

 mus are still practically identical. The Tampa and San Diego Pectens, while still 

 easily distinguishable, have not differentiated so far that their specific proximity is 

 obscured. The present dissimilarity between " ventricosus " and "gibbus" is in 

 fair harmony with the prevailing view of former oceanic connections. 



V. THE SPECIFIC IDENTITY OF PECTEN VENTRICOSUS. 



I do not propose to discuss this point in any detail, but I may point out that 

 the common littoral Pecten of Cold Spring Harbor (PI. IX, Fig. 5) is as easily dis- 

 tinguished from that of Tampa as the latter is from that of San Diego. Indeed, 

 in many respects, such as details of color markings and number of rays, the Tampa 



