286 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



vol.. XXIII. 



1. SUBTROPICAL SPECIES. 



PACIFIC COAST. 



Tellina cumingi. 

 Merisca redusa. 

 Merisca crystallina. 

 EfliptoteUinri pacifica. 

 Euryiellina rubescens. 

 Moerella Tnerojms. 

 Angulns mamel/ii. 

 Angulus carj)enteri, 

 Angulus modestus. 

 Scissula virgo. 

 Strigilla fucata. 

 Strigilla clcercida. ■ 

 Strigilla lenticula. 

 Tdlidora bumeti. 

 Metis alia. 



Macoma leptonoidea. 

 Cymatoica undulata. 

 Psammacoma iMnamensis. 



ATLANTIC COAST. 



Tellina interrupta. 

 Merisca lintea. 

 Merisca crystallina. 

 Elliptotellina americana. 

 Eurytellina angidosa. 

 Angulus promerus. 

 Angulus sybarilicus. 

 Angidus consobrinus. 

 Angulus tener. 

 Sci'tsida exilis. 

 Strigilla camaria. 

 Strigilla pisiformh. 

 Strigilla flexuosa. 

 Tellidora cristata. 

 Metis intastriata. 

 Macoma leptonoidea. 

 Cymatoica orientalis. 

 Psammacoma extenuata. 



2. BOREAL SPECIES. 



PACIFIC COAST. 



ATLANTIC COAST. 



Macoma krausd. 

 Macoma calcarea. 

 Macoma balthica. 



Macoma krausei var.f 

 Macoma calcarea. 

 Macoma balthica. 



Table 1 shows that of the subtropical species eighteen are repre- 

 sented to some extent on both sides of the Isthmus of Panama and 

 Middle America. Of these two, or perhaps three, are unchanged ])y 

 their long separation, or about 17 per cent. If we adopt the geolog- 

 ical percentages l)y which faunas are referred to suljclivisions of the 

 Tertiar}", this proportion would indicate for our Tellinas a separation 

 dating from some time in the Miocene, which is exactly what w^e learn 

 from the geology of the Middle American region, where the last 

 marine beds of any general extent, indicating a connection of the two 

 oceans, belong to the Oligocene epoch, while the absence of marine 

 Miocene from the whole of this region leads to the belief that the land 

 surface was during that epoch above the sea. 



I do not regard the evidence of the Tellinas alone as more than a 

 trifle, but such as it is it coincides with other evidence of more weight. 

 While some portions of Middle America may have been low enough 

 to permit the passage of water between the two oceans since the Upper 

 Oligocene, yet it is quite certain that t\A^ connection, if it existed, did 

 not lead to an}^ important interchange of animal life, nor prevent the 

 northward migration of South and Middle American vertebrates and 

 fresh- water raollusks into the continent of North America. 



The boreal types tell another story, as they are practically common 



