508 FIBULARIA OVULUM. 



it has not been examined critically with a view to determining its affinities ; 

 most of the specimens usually preserved have lost their spines, as well as the 

 anal and buccal membranes. The genus is retained among the Clypeastroids 

 from its close affinity to Echinocyamus. The presence of teeth and of par- 

 tial rudimentary partitions only shows that the genus is an anomalous one, 

 and its exact position either with the Clypeastroids or Cassiduloids cannot 

 be accurately determined until we are better acquainted with the anatomy 

 of Fibularia than we are at present, as there are as many reasons for 

 associating them with the one group as with the other; and until we can be 

 sure that neither Fibularia vulva nor F. ovulum are not young stages the 

 question must remain unanswered. 



The test is ovoid, angular, pointed anteriorly, high ; abactinal surface regu- 

 larly arched ; petals short; pores diverging, distant, not conjugate ; poriferous 

 zones consisting of not more than six or seven pairs of pores in specimens 

 measuring l()" m ', longitudinal diameter. Middle of interambulacral plates 

 raised, forming ridges running from the apex to the actinostome, especiallv 

 well developed in the anterior part of the test. Apical system central ; 

 vertex anterior, at the extremity of the odd petal ; test sloping, uniformly 

 arched towards the posterior vi]^ ; petaliferous part of test flattened. 

 Lower surface convex, mouth and anus alone being placed in a slight de- 

 pression. Actinostome small, pentagonal ; bourrelets and phyllodes very rudi- 

 mentary, — the former reduced to small smooth interambulacral spaces, the 

 phyllodes to a couple of large buccal pores. Anal opening narrow, longi- 

 tudinally elliptical, half-way between the edge and the actinostome ; does not 

 equal in length the diameter of the mouth. The greatest diameter of the 

 test may be either anterior or posterior to the apical system, the outline from 

 above and in profile being extremely variable. Tuberculation regular, dis- 

 tant ; miliaries closely packed. 



There is great difference in the size of the jaws in the two species of Fibu- 

 laria; in F. volva they are exceedingly small, scarcely projecting beyond the 

 auricles, while in F. ovulum they are large and more prominent, much as we 

 find them in Echinocyamus. The peculiar pits between the radiating ridges, 

 so prominent in the plates of the interior of Echinocyamus, exist also in all 

 the species of Fibularia, but are less marked ; they are more prominently 



