156 



and provided with radiating furrows, which correspond with llie radiafely arranged 

 zott'cia. 



Similar vibracula are found in the cretaceous species Rhagasostoma elegans 

 V. Hag. 



Family Cribrilinidae. 



(I'l. IX, ligs. 9-11). 



The zoa'cia with a larger or smaller, membranous i'ronlal area, covered by 

 two rows of mutually coalesced, hollow, marginal spines, which form a frontal 

 shield perforated by slits or pores. 



The unnaluralness of this family may he sufficiently evident from the fact, 

 that it is only based on a single character, which has moreover been taken from 

 formations as variable and as inconstant in appearance as the spines. It is true 

 that all such forms, in which the spines by their mutual connection form a shield 

 broken through by slits or pores, have a certain outward similaritj', whicli without 

 a close examination may easily be considered a proof of real relationship. The 

 fact is however that we might be equally justitied in forming a family for all 

 such Menibraniporidae, in which spines are absent or for such as possess two rows 

 of well-developed, unconnected spines. In reality we do not in the other structural 

 features find such a degree of conformity as might justly be expected in a natural 

 family. A careful examination leaves no doubt of the independent origin of such 

 a frontal shield in many difi'erent forms. That Ilarmer has a similar conception 

 of these forms, is evident from the following statement of his: >The existence of 

 great differences between the opercula of dilferent species at present referred to 

 Cribrilina suggests that the genus is an unnatural one, representing a stage of 

 evolution of the Lepralioid zooecium, which has been arrived at independently 

 in several cases.- ' 



Electro monostachys (PI. IX, figs. 2 a — 2 b) and K. anyiilutu n. sj). (PI. XXII, 

 fig. 4 a) constitute two of the best examples of the inconstancy of the spines. 

 Here we may find in the same colony some zocrcia, which are entirely without 

 spines, and others provided with a larger number of these structures. In the face 

 of this fact it would surely be impossible to nuike the i)resence or absence of 

 spines the only dislinction between two systematic sections. We are however able 

 to mention two quite corresponding examples of the inconstancy of the frontal 

 shield, viz, besides the above-mentioned Electra zostericola a new species from the 

 Foeroes which is related both to Callopora Diimerili and to Menibrdiiiiiorella nilidu. 



' 19, p. 329. 



