PLEUROBRANCHUS AGASSIZII MAC FARLAND 59 



Pleurobranchus agassizii Sp. Nov. 

 Plates XI and XII, Figs. 43-57. 



Three small specimens of a Pleurobranchus were taken by 

 Mr. Greeley at Riacho Doce, Alagoas. They were killed in 

 formalin and afterward transferred to alcohol. The coloration 

 of the animals in life was not noted; the color of the preserved 

 specimens is a rather uniform pale, pinkish yellow. In two of the 

 specimens a fine light brown mottling seemed to divide the dorsum 

 into very minute polygonal areas, but even this trace of color 

 gradually disappeared on their being transferred to alcohol. 



EXTERNAL CHARACTERS. 



Size. The three individuals measured 10, n and 8 mm. in 

 total length, by 6, 6.5 and 5.0 mm. in width respectively. In each 

 case the foot is somewhat contracted, the mantle but slightly so, 

 the measurements in life probably exceeding the above somewhat. 

 The length of the foot is 6.5, 6.0 and 5.5 mm., with corresponding 

 widths of 4.0, 4.0 and 3.0 mm. in each case. 



Body form. The body is arched, slightly depressed, oblong ; 

 the mantle broad, extending far beyond the foot throughout its 

 entire circumference, though the strongly contracted posterior 

 end of the foot, the rhinophores, and the frontal veil probably 

 extend well beyond the mantle margin in the living animal. The 

 mantle margin is very slightly emarginate above the tail. The 

 surface of the dorsum is smooth, save for slight, irregular nodo- 

 sities formed by unequal contraction. The mantle margin is 

 moderately thick and very wide, being 2.0 mm. in width, its free 

 edge being smooth. 



Shell. The white calcareous shell (PI. XI, fig. 43) shows 

 plainly through the mantle in all the specimens. It is placed well 

 forward, its anterior margin being above the head region, while 

 the posterior portion covers the anterior two-thirds of the poste- 

 rior visceral mass. In outline it is oblong, nearly linear, the 

 lateral margins being nearly parallel. The anterior margin is 

 more gently rounded than the posterior one, the spire very small, 

 oblique, the whole shell being made up of about two turns, the 

 outermost one very broad and flat, and forming almost the whole 

 area of the shell. The lines of growth are plainly marked; the 



