126 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



We have collected a dozen dead specimens none of which had 

 the onychocellarium. The absence of this organ as well as the 

 special form of the ovicell are perhaps the characteristics of a differ- 

 ent genus. Our material was not sufficient and we maintain this 

 species in Onychocella because of the general form of the opesium. 



Occurrence.— D. 5478. Pacific, Tacbuc Point, Leyte; 10° 46' 24" 

 N.; 125° 16' 30" E.; 57 fathoms; sh. 



Cotypes.— Cat. No. 7923, U.S.N.M. 



Genus RECTONYCHOCELLA Canu and Bassler, 1917 



We now know numerous recent species of this important genus 

 which differs from Velumella Canu and Bassler, 1917 only in its 

 rarely visible opesiular indentations, these corresponding to the 

 opesiular muscles placed low and to less calcification of the frontal. 

 We have noted the same phenomenon in Dacryonella. In the equi- 

 torial zone this is a genus of depths from 296 to 622 meters with a 

 temperature of 12.4° to 13° Centigrade. In the northern part of 

 the temperate zone it is found at 210 meters but it is very rare here. 

 It is an equatorial genus especially, little sensible to variations of 

 temperature of the pelagic depths. The following species can be 

 cited for this genus. 



Rectonychocella (Smittipora) abyssicola Smitt, 1872 



(fig. 61) Recent, Gulf of Mexico. 



Rectonychocella (Onychocella) solida Nordgaard, 1907 Recent, Atlantic. 



Rectonychocella ovalis, new species Recent, China Sea. 



Rectonychocella grandipora, new species Recent, Philippines. 



Rectonychocella bilamellaria Canu and Bassler, 1920 Eocene, United States. 



Rectonychocella semiluna Canu and Bassler, 1920 Eocene, United States. 



Rectonychocella tenuis Canu and Bassler, 1920 Eocene, United States. 



Rectonychocella elliptica Canu and Bassler, 1920 Eocene, United States. 



Rectonychocella (Eschara) didyma D'Orbigny, 1852 Cretaceous, Europe. 



Rectonychocella (Biflustra) ligeriensis D'Orbigny, 1852. _ Cretaceous, Europe. 



An important biological observation to be noted is that the species 

 of Rectonychocella resist the- pressure of the great depths better than 

 the species of Velumella. The bathymetric changes could therefore 

 in that group of animals be the causes of variations and of specific 

 differences. It is in error then that some paleontologists attribute 

 the variations of species only to the contraction of the tropical zone. 



The paleontological interpretation of these observations is very 

 interesting. The disappearance of the genus in the Vicksburgian 

 indicates an almost total elevation of the sea bottom. This phenom- 

 enon must have occurred slowly and progressively for considering 

 the size of the zooecia, the upper Jacksonian of the Chipola River and 

 Sepulga Itiver localities indicate water of less depth than the Middle 

 Jacksonian of Wilmington, North Carolina, and the largest species 

 was moreover from the Lowor Jacksonian of Jackson, Mississippi. 



