BRYOZOA OF WOODS HOLE REGION. 229 



versely. Seen from the dorsal side the zooecium shows two pairs of lateral pore chambers and a single 

 large anterior one, with spines projecting into the chambers. 



This species appears to be rare in this region, and in the course of several years of dredging and other 

 collecting in this group I have found it only a few times. Gay Head on Devils Bridge reef, 2 to 3^^ 

 fathoms; Woods Hole harbor on Fucus, Vineyard Haven, and Nantucket Harbor on algae, attached to 

 piles; Crab Ledge, 15 fathoms on stones and shells. 



As stated under that species, Leidy's and Verrill's records of lineata refer rather to monostachys. 



Membranipora craticula Alder. [PI. xxiii, fig. 32, 32a, 32b.] 



Alder 1857. p. 144. 



Verrill 1879c, p. 29 (as M . lineata var. craticula). 



Whiteaves 1901, p. 96. 



Zoarium forming small, usually rounded patches, encrusting shells and stones, rarely on algse. 

 Zooccia small, arranged usually in radiate series; the membranous area is somewhat elliptical in outline, 

 the raised margin broad and provided with about 14 long spines. The most anterior two pairs are longer 

 and blunter than the others; the first pair is directed well forward, the second pair more erect; and the 

 remaining ones, wiiich are somewhat flattened in cross section, bend forward and downward over the 

 area in a very characteristic manner. On the dorsal side the zooecium strongly resembles M. lineata, 

 but there are no spines projecting into the pore chambers. The avicularia are larger than those of 

 A/, lineata and are raised upon a bulging prominence (dwarf zooecium ?). The ooecia are large, roimded, 

 smooth, and shining, with a raised rib much as in M. lineata, except that it is more uniformly bent back- 

 ward at its middle. 



Not uncommon in the outer waters of the region, but not noted in Vineyard Sound or Buzzards Bay. 

 Taken at Muskeget Channel, Great Round Shoal, Crab Ledge, No Mans Land, and Nantucket Shoals. 

 The species is a northern one and has not hitherto been recorded south of Canadian waters. 



Membranipora arctica (d'Orbigny). [PI. xxrn, fig. ^t,, 33a, 33b, 34, pi. xxx, fig. 86.] 



D'OrbiEny 1851, p. 571 (Rcploflustrella). 



Verrill 1879c, p. 29 (as M . unicornis var. sopkitc). 



Wliiteaves 1901, p. 96-7 (M. sopkicB and the var. armifera Hincks). 



Zoarium forming more or less circular, grayish or brownish patches on shells and stones, rather 

 coarse, often an inch or more in diameter. Zooecia large, the membranous area oval or sometimes nearly 

 round, the margin furnished with 4 to 6 very stout flattened spines, which are often somewhat contracted 

 at the base and which bend down closely over the area. A pair of small avicularia (occasionally only 

 one) on either side of the opercular opening, with an obtusely pointed mandible directed forward and 

 somewhat toward the midline. Ovicell short, broad, and flattened, in the older parts of the colony 

 deeply immersed, crossed by a raised rib. 



In the younger stages a relation is shown to .1/. lineata ;ind M. craticula, but arctica is a much coarser 

 species; in older stages when the spines are heavily calcified there is a superficial rcsembhmce to species 

 of Membraniporclla. 



Not uncommon at Crab Ledge on shells and stones, 14 to 20 fathoms. Not previously recorded 

 south of the St. Lawrence. 



A well-marked variety of this species, var. armifera, plate xxiii, figure 34 (Hincks, 1880b, p. 82, Mem- 

 branipora armifera), occurs at Crab Ledge with the typical arctica, but it is less common. The general 

 character of the colony and of the zooecia is much the same as in the tj-pical arctica, but it differs in the 

 more slender character of the spines, in the presence of a small erect spine situated partly within the 

 margin, just behind and in close relation to the avicularia on one or both sides of the aperture. A large 

 elongate aviculariura is sometimes present at the base of the zooecium behind the aperture, situated 

 on a raised base which often overlaps the ovicell of the cell behind so as to appear a part of it. More 

 often the raised base alone is present without an avicularium. Hincks figures the avicularium as 

 pointing forward alongside the aperture, but it may be turned in any direction. He also describes the 

 small lateral avicularia as pointing outward and backward, but in our specimens this condition is seen 



