30 





posterior. Valves generally thin and pellucid, with the surface in some cases 

 quite smooth, in other cases closely punctate or reticulate, lower edges scarcely 

 serrate, but, as a rule, fringed with very small and delicate spinules. Anterior 

 antennae each composed of 4 rather unequal joints, the first 2 very obliquely 

 connected with each other, 2nd joint considerably dilated at the base and filled 

 with strong muscles acting upon the small terminal part of the antenna, its 

 upper edge provided in the middle with a slender anteriorly-pointing seta; 

 3rd joint very movably articulated to the preceding joint and carrying a small 

 bristle at the end anteriorly; last joint so extremely small as only with diffi- 

 culty to be discerned, and tipped with 4 exceedingly long and slender subequal 

 natatory setae. Posterior antennae with the outer ramus 8 9 articulate, and 

 resembling in structure that in the Myodocopa; inner ramus not much shorter 

 than the outer and 3-articulate, with the 1st joint naked and much larger than 

 the other 2, which are densely clothed with natatory setae of the same appea- 

 rance as those on the outer ramus. Anterior lip rounded in front. Mandibles 

 with the cutting edge divided into 4 or 5 unequal teeth, proximal joint of palp 

 forming behind a rounded lobe clothed with 4 plumose setae, distal joint blunted 

 at the end, and provided with 4 slender apical setae arranged in pairs. Maxillae 

 with the rami of about equal length. Maxillipeds with the terminal joint well 

 defined and tipped with a single seta. Caudal lamellae in female without any 

 denticles of the anderior edge; left lamella in male sending off from the base 

 anteriorly a more or less produced spiniform process adjoining the copulatory 

 apparatus. 



Remarks. Of this remarkable genus at first only a single species was 

 found, described by the present author in the year 1865 under the name of P. 

 orbicularis. In the next following years, however, 2 other nearly-allied species 

 were added, and in recent times the number of species has been considerably 

 increased by other authors. Thus G. W. Muller records from the Mediterranean 

 no less than 9 different species referred by him to this genus. Some of these, 

 in particular the 3 species, P. dentata, rostrata and tuberosa, seem however 

 to me to differ so conspicuously from the typical species, both in the general 

 appearance and in the rough sculpture of the shell, that they perhaps more 

 properly should be placed within a separate genus. On a closer examination 

 of the material in my possession, I have been enabled to increase the number 

 of Norwegian species referable to the present genus to 6 in all. 



