198 PROCEEDINGS OP THE ACADEMY OP [1884. 



some distance beyond the surface of the coenoecium. The total 

 length of the digestive tract is rather less than that of the 

 lophophoric arms and about equal to that of the outer rows of 

 tentacles. These are fewer in number than in any other described 

 species, ranging from 52-60.^ In the great majority of the 

 polyp heads which have been examined the number was 54 ; far 

 less frequently they range upward through 56 and 58 to 60, in 

 only one instance passing that number. On the other hand the 

 tentacular hooks of the statoblasts are more numerous than in 

 G. ophidioidea, and about the same as in the other species. 



Three species of the genus have been already described, G. 

 mucedo, Cuvier, in Europe, and G. Idee, Leidy, and G. ophidioidea, 

 Hyatt, in America, The differences existing amongst them are 

 not considerable, and it admits of question whether all should 

 not be merged under the prior title. In the present condition of 

 the subject it would seem that the species now brought forward 

 is at least as clearly differentiated from any of the former ones 

 as they are from each other. I will therefore name it, provision- 

 ally, Gristatella lacustris. 



EXPLANATION OF PLATE IV. 



Fig 1 represents a transverse section through the centre of a statoblast of 

 this species, Gristatella lacustris ; a, a, the exposed chitinous surfaces 

 of the valves ; b, b, the reflexed ; c, c, the bent, incurved retentive 

 hooks ; d, d, section of the annuhis, or ring of air cells surrounding 

 the chitinous body of the statoblast ; e, e, the part of the rim at 

 which the valves separate at the time of germination, as is shown on a 

 larger scale in 



Fig. 2, which represents one end of the section of a similar statoblast in 

 the act of separation, the parts indicated by letters corresponding to 

 those on fig. 1, with the addition of /, a delicate film which is being 

 stripped from the under siirface of the annulus, and g, (/, which suggest 

 the relative sizes and frequency of the papillae upon the exposed 

 surface of the valves. 



Fig. 3 exhibits for comparison a corresponding section of the statoblast of 

 Pectinatella magnifica, Leidy, lettered as before ; a, a, the exposed 

 surface of the valves ; b, b, the single series of anchorate hooks ; 

 d, d, sections of the annulus, itself divided by the line e, e, along 



^ In C. mucedo and C. Idee these are said' to be "about 80." In C 

 ophidioidea, "not above 90." 



