DALL: MOLLUSCA AND BRACHIOPODA. 209 
that the separation between the abyssal fauna of the Pacific and that of 
the Antilles is very ancient indeed, for in the shallows many of these 
groups are represented on both sides of the isthmus of Darien, yet have 
not succeeded in reaching the deep water. That they can endure life 
in the abysses and are not kept out by any physiological character- 
istics which unfit them for life in deep water, is proved by the fact that 
on one side or the other they do at present flourish there, though not on 
both sides. 
The peculiarities of distribution, when analyzed, prove remarkably 
interesting and not a little puzzling. 
For instance, the following Antillean groups, some of which are very 
populous in species, are not revealed in the Pacific dredgings from deep 
water : 
Ovulactaeon Aurinia 
Ringicula Turbonilla 
Trigonostoma Addisonia 
Benthobia Microgaza 
Conomitra Dillwynella 
Mitra Basilissa 
Nassarina Pleurotomaria 
Dalium Fissurisepta 
Eudolium Pectinodonta 
Triforis Lepetella 
Cerithiopsis Hanleyia 
Seila i Limaea 
Mathilda Spondylus 
Fluxina Dimya 
Clathrella Astarte 
Rissoa Liocardium 
Benthonella Meiocardia 
Hyalorisia Abra 
Olivella Euciroa 
Marginella Thecidium 
Voluta Megathyris 
The above list includes the more conspicuous instances only. 
On the other hand the Pacific fauna has the following groups not rep- 
resented in the Antillean deep sea : 
Trenosyrinx Surculina 
Steiraxis Narona 
Jalliotectum Adelomelon 
Borsonella Miomelon 
Phymorhynchus Tractolira 
VOL. XLIII. — NO. 6 14 
