10 MICROSCOPICAL EXAMINATION 



Genus Dentalina, D'Orb. Shell much elongated, shghtly arcuate, smooth, 

 slender, formed of long oblique cells, gradually increasing in size ; aperture 

 small, round, in the acuminated end of the last cell. 



Dentalina mutabilis, Bail., (fig. 7.) Shell composed of slightly convex smooth 

 cells, which are variable in length, a short one often succeeding a longer one ; 

 aperture surrounded by radiant ridges. Several fragments were found in the 

 deep soundings labelled No. 1, H, 90 fathoms. 



Genus Marginulina, D'Orb. Shell free, regular, equilateral, elongated, ar- 

 cuate, often curved at the posterior extremity in form of a crosier, formed of 

 globose partially envelloping cells, the last of which is always convex, and often 

 prolonged ; the first cells turned backwards, and often showing an approach to 

 a spiral arrangement ; imaginary axis arcuate, the convexity on the same side 

 as the opening ; aperture rounded, usually placed at the extremity of an elonga- 

 tion of the last cell. 



Marginulina Bacheii, Bail., figs. 2, 3, 4, 5, and 6. Shell elongated, smooth, and 

 shining, formed of cells, the first of which are arranged in a spiral manner, and 

 compressed laterally ; the others gradually assume a more globular form, and an 

 oblique position, giving to the whole shell a sigmoid form ; aperture at the elon- 

 gated extremity of the last cell, surrounded by small ridges. 



This fine species is one of the largest and most conspicuous forms in these 

 soundings. It was found in considerable numbers in all the soundings except 

 the shallow ones, from S. E. of Montauk Point to S. E. of Cape Henlopen. 



I take pleasure in dedicating this species to Prof A. D. Bache, Superintendent 

 of the Coast Survey. Fig. 2, side view of the young shell ; fig. 3, front view of 

 the same ; fig. 4, end view of the same, showing the orifice ; fig. 5, the full grown 

 shell ; fig. 6, end view, showing the orifice. 



Genus Robulina, D'Orb. Shell free, regular, equilateral, suboi'bicular, much 

 compressed, carinate, vitreous, shining, formed of elongated cells, constantly 

 arranged in an envelloping spiral, and uniting in the region of the umbilicus. 

 Aperture triangular, longitudinally cleft, situated at the carinal angle of the cells. 



Robulina D'' Orbignii, Bail., (fig. 9, 10,) shell discoidal, compressed, with a 

 narrow but sharp carina, surface free from ridges, cells about eight in number, 

 with traces of the successive apertures visible on the three or four last. Aper- 

 ture a longitudinal cleft surrounded by small ridges. It occurs in considerable 

 ilumbers at all the localities mentioned as furnishing the Marginula Bacheii. I 

 dedicate it to Alcide D'Orbigny, who, by his labors, may be said to have created 

 the important and interesting branch of science to which these minute shells 

 belong. 



Genus Rotalina, D'Orb. Shell free, depressed or trochoid, minutely perfor- 

 ated, often carinate, formed of a depressed spire, truncate or conical, composed 

 of depressed cells, pierced with an aperture in the form of a longitudinal cleft, 

 on the side of the penultimate turn of the spire, occupying but a portion of the 

 last cell. 



Rotalina Ehrenbergii, Bail,, figs. 1 1, 12, 13. Shell orbicular, depressed, nearly 



