448 BULLETIN OF THE 



branchial areas are composed of lamellae, between which at their bases are 

 narrow fissures bridged longitudinally by slender fibres which act as regulators. 

 In this species the two areas are similar, and resemble those of Cetomya tornata 

 Jeffreys, as figured by Pelseneer. 



In Poromya sublevis Verrill, dredged by the Albatross, in 1685 fms., off 

 Chesapeake Bay, by carefully dissecting away the septum, which presented 

 much the same appearance as that of F. granulata in Pelseneer's diagram 

 (op. cit., pi. iii. fig. 7), several interesting facts were disclosed. The posterior 

 lamella3 were not separated by fissures at their base. This seemed evident on 

 an external view, but was made more certain by an inspection of the upper 

 surface of the septum, where these openings, when they exist, are always con- 

 spicuous. The anterior areas were fissured, especially near the foot, but less 

 so behind, so that when I first examined this species, taking the extreme deli- 

 cacy of the membranes into account, and the apparently imperforate character 

 of the posterior areas, I suspected that the fissures were due to tearing or in- 

 cautious probing. A reversal of the septum, and an examination of other 

 species showed, however, that there are variations in this respect, and that 

 Pelseneer had correctly described the conditions which exist in some of them. 

 An interesting feature disclosed by the examination of the septum under trans- 

 mitted light was, that the blood-vessels which supply the branchial lamellse 

 appear to reach them from behind, a separate vessel starting from the vicinity 

 of the siphons, and running a somewhat irregular course to each of the lamellar 

 areas on each side. There seemed to be no continuation of these vessels an- 

 teriorly in front of the areas which they serve. The ovisacs are not lobulated, 

 as in Myonera, but more evenly spread over the posterior surface of the visce- 

 ral mass. The ripest eggs were large and conspicuous. There was no evidence 

 of their extrusion through the covering of the visceral mass, as in Myonera, 

 though this may take place later. 



Cetomya alhida (I. p. 282). In young specimens of this and other species, 

 the membranes of the septum, etc. are extremely delicate. The use of too 

 concentrated hardening agents, or the incautious touch of a probe, will produce 

 lesions which may be indistinguishable from normal fissures. To make sure 

 that nothing of this sort shall happen, it is necessary to float the soft parts in 

 a cup of water, and turn them about with delicate forceps. This is not con- 

 venient in all respects for observation, but with time and patience the charac- 

 ters may be made out. 



Young specimens of this species show the lamellar areas as usual, with the 

 depressions above them in the floor of the upper chamber, but the fissures are 

 not open; a fact which leads me to believe that they appear only with matu- 

 rity. A very delicate membrane seems to hold the distal margins of the 

 lamellae together, so that a delicate probe passes over without separating them. 

 It was only after long experimentation and study of the specimens that I be- 

 came convinced that this is the case, although there is no inherent improba- 

 bility that it should be so. 



Poromya (Dermatomya) mactroides n. s. This fine species difi'ers from the 

 May 20, 1889. 



