230 BULLETIN OF THE BUREAU OF FISHERIES. 



only in the infertile cells near the center of the colony, while in the zooecia of the same colony bearing 

 ovicells they are directed forward and inward, as in the typical form. 



Membranipora unicornis (Fleming). [PI. xxm, fig. 35.] 



Fleming 1828, p. 536 (Flustra unicornis). 

 V crrill 18790, p. 29. 

 \Vhiteaves 1901, p. 96. 



Zoarium encrusting shells, forming rounded whitish colonies. Zooecia large, somewhat translucent, 

 the surface shining; aperture large, oval, somewhat contracted at the anterior end; the margin broad, 

 finely crenulate on its inner edge, and bearing usually four spines near the forward end. The anterior 

 pair of spines is small and erect (often wanting); the other pair is larger and usually unequal, one being 

 much longer than the other, and the longer one may stand nearly erect or bend somewhat over the 

 aperture. The ovicell is large, smooth, and bears a transverse rib. An avicularium is usually present 

 at the base of the zooecium, mounted on a raised projection; when the ovicell is present the avicularium 

 appears to arise from the ovicell, and the mandible is directed somewhat forward, but when the ovicell 

 is absent the avicularium is reversed in position, the mandible pointing backward. 



Dredged at Great Round Shoal in 8 fathoms, a number of fine colonies. Not hitherto recorded south 

 of Canadian waters. 



Membranipora cymbaeformis Hincks. [PL xxm, fig. 36, 36a.] 



Hincks 1877, p. 99, no, 149; 1888, p. 217 (M. cymbiformis). 

 Verrill 18790, p. 29 (\I. spinifera). 

 Whiteaves 1901, p. 96. 



Zoarium encrusting the stalks of hydroids, Bryozoa, etc., usually forming small colonies of a very 

 irregular form. Zooecia large, deep, with unusually high walls; the aperture is large and is often slightly 

 bridged over, especially near the base, by a secondary lamina; the margin, which is rather broad, bears 

 about 6 or 8 long erect spines, and usually one or two long pedicellate avicularia which occupy the same 

 position as spines. Ovicells wanting. 



Crab Ledge, 14 to 20 fathoms, and off Sankaty Head, ESE, 13 to 20 fathoms, rather common. 

 The colonies are never large, and the best I have seen for study have been on the back of Bugula 

 murrayana. 



Membranipora aurita Hincks. [PI. xxm, fig. 37, 373, 37b.] 



Hincks 1877, p. 213. 



Zoarium encrusting, usually on shells, but often on algae. In the former situation circular colonies 

 are produced and the zooecia are often disposed with extreme regularity, but on the stems of algae they 

 are generally irregular and the cells sometimes crowded. Zooecia moderately large, considerably 

 narrowed at the anterior third, the walls high, and in the older colonies strongly calcified; entirely 

 membranous in the younger stages, but partly closed by a calcareous lamina when fully calcified (Hincks 

 describes it as entirely membranous); margin broad, finely tuberculate on its inner side, beset with one 

 to four spines which are more or less erect (usually only one or two are found in the adult, and where 

 two are present one is much larger than the other). Ovicell rounded, more or less immersed according 

 to age and calcification, bearing a strong raised rib, which encloses a triangular space on the front of the 

 ooecium and which often rises into a strong umbonate process at the top in old colonies. The avicularia 

 vary in a remarkable manner according to whether an ovicell is present on the zooecium just posterior 

 When 110 ovicell is present a single avicularium is regularly present with its tip usually pointing back- 

 ward, but when the ooecium is present there are very constantly two avicularia placed with great 

 regularity on either side of the ovicell and pointing forward and outward. Just how the development 

 of an ooecium, which belongs to another cell, should thus influence the number and position of the 

 avicularia is by no means clear. 



Rather common and well distributed, found in Buzzards Bay and Vineyard Sound, Muskeget Chan- 

 nel, Great Round Shoal, and Crab Ledge, dredged in 3 to 18 fathoms. The species is known from 

 England and Denmark, but has not hitherto been reported from American waters. 



