FIBULAUIIDAE. 55 



blades (PI. 124, fig. 21), the only ones found may be called either small tridentate 

 or triphyllous. Their valves (PI. 124, fig. 22) have the blades very broad and 

 the margins very coarsely dentate. As these valves are scarcely .10 mm. in 

 length, I have preferred to call them triphj'llous. Australia and Tasmania are 

 the home of this interesting species. 



FIBULARIIDAE Gray. 



It is of course difficult to determine whether the .simple nature of the petals 

 and madreporite in the members of this family are primitive characters retained 

 or secondary simplifications. From the geological point of view, this group ought 

 to be the ancestral stock from which the other clypeastroids have been derived, 

 it is known from so much earlier strata than any of the others. But if we dis- 

 regard this single fact, it becomes evident that nothing in the structure of the 

 Fibulariidae requires us to consider them as primitive, wliile many details suggest 

 that they are highly specialized. Thus there can be no question that the condi- 

 tion of the auricles and the arrangement of the aboral interambulacral plates 

 are both specialized, and it seems equally clear that the structure of the miliary 

 spines is of the same nature. These three characteristic features ally the family 

 very closely with the Laganidae, while the two characters which separate the two 

 famihes, the condition of the petals and madreporite, cannot be positively placed 

 as either primitive or derived. Hawkins (1912, Geol. Mag., new ser. dec. 5, 

 9, p. 297) has recently described a Fibularia from southern Nigeria, which is 

 very interesting in considering this matter, for it has well-differentiated petals 

 and numerous madreporic pores. Unfortunately Mr. Hawkins has not been able 

 to determine the condition of the auricles. The geological horizon of this 

 species is "probably Lower or Middle Eocene." It is possible to interpret this 

 as one of the early, primitive species of the family from which our modern 

 Fibulariidae have been derived by the reduction of the petals and loss of all 

 madreporic pores but one. Except for the shape of the test and the unusually 

 divergent poriferous areas, Fibularia nigeriae is just as much one of the 

 Laganidae as it is one of the Fibulariidae. It seems therefore that the discovery 

 of F. nigeriae strengthens the belief that the Fibulariidae are a modern, highly 

 speciahzed family nearly related to the Laganidae, and probably derived from 

 the same stock. 



The familj' contains only two genera and these are not to be distinguished 



