Polyzoa of the St. Lawrence. 217 



approach is Flustra Ar-agoi of Aurlouin 5 but apart from other 

 differences the latter is at once distinguishable by the peculiar 

 structure of the oral extremity of the cell and the remark- 

 able lobed appendage on each side of the orifice. It 13 

 difficult to meet with a perfectly formed cell of M. crassicosta. 

 Amongst the St.-Lawrence dredgings it is far from uncommon, 

 and I have liad the opportunity of examining a good many 

 specimens ; but scarcely a cell has occurred in which the full 

 complement of ribs was present. In a large proportion ot 

 cases only the oral ring and the first pair of the rib-like pro- 

 cesses are developed, so that the colony presents a most un- 

 finished appearance. The ribs are flat and massive and 

 expanded at the base ; their points meet in the centre and 

 unite without much regularity, and there is consequently no 

 Btraight median line of junction, as in the British M. nitida. 

 In the early stages of development the cell is destitute of all 

 covering but the membranous front wall, and is furnished 

 with a plain thickened margin, on which no trace of the rib- 

 like spines is yet visible. The first to appear are the two 

 oral appendages, which originate on each side just below the 

 top, and as they increase in length bend round towards the 

 front and meet in the centre, forming the oral margin, on 

 each side of which there commonly rises a mucronate projec- 

 tion. The first pair of ribs are developed immediately below 

 the margin^ to which they are generally closely united 

 throughout a large portion of their length. A second pair 

 originate lower down on each side, and pass diagonally 

 towards the centre, where they unite with the others ; and 

 usually one or two ribs more pass upward from tlie lower 

 margin towards the same point. This is the general plan, 

 but there are many irregularities. The ribs are for the most 

 part separated by rather large lacuna3, so that in this case the 

 protective covering is an open framework and not a solid wall. 

 The reddish-brown colour of the crust seems to be a constant 

 character in fresh specimens. 



Other species of this genus are M. nitida and M. vielolontha 

 (Brit.), M. diatans (Austr.), M. sceletos (Madeira), M. Ajas- 

 sizii (l^^lorida), M. Aragoi (? Medit. or Ked Sea). 



Family MembraniporidsB. 

 Membranipora, De Blainville. 



Memhrainpora cymhiformis^ Ilincks. (PI. XV. figs. 4) 



Membranipora spinifera, Smitt, Ki'it. Forteckn. ofver Skandin. Ilafs- 

 Bryoz. pt. 3, pi. xx. fig. 32. 



This form was first noticed by Smitt, but he referred it to 



