i88 



FOSSILS OF DOUBTFUL AFFINITIES. 



opinion as to its identity with Nidulites. It is clear, however, that 

 the two are, at any rate, very closely related, if not absolutely iden- 

 tical. The name Nidulites was proposed by Salter for certain sin- 

 gular Silurian fossils (subsequently described by Eichwald under 

 the name of Mastopora), which have the form of ovate, glob- 

 ular, or pyriform, hollow bodies, with a thin calcareous external 

 wall, and probably attached to foreign bodies by a peduncle (fig. 

 73, i). The wall of Nidulites encloses a large central space, and is 



formed of short, wide, hex- 

 agonal calcareous tubes or 

 cells, which are united to- 

 gether at their basal ends and 

 also by their walls (fig. 74, a 

 and b). The outer ends of 

 these shallow tubes or cells 

 are open, while the inner 

 ends are closed by curved 

 basal plates, the convexities 

 of which are turned towards 

 the central cavity of the or- 

 ganism. Moreover, the closed 

 end of each cell is perforated 

 by a small round central aper- 

 ture (fig. 74, c), so that each 

 cell communicates freely with 

 the great internal cavity. 



With regard to the affini- 

 ties and systematic position of 

 Nidulites and Cyclocrinus, it 

 is not possible at present to give any decided opinion. There is no 

 positive character exhibited by these singular fossils which would 

 enable us to refer them definitely to any known division of the 

 animal kingdom. The resemblances which they present to the 

 Receptaculitidce or to the Cystideans are quite superficial, and it is not 

 even clear that they are genuinely related to the equally enigmatical 

 genus Pasceolus. It may be pointed out, however, that there are some 

 curious points of resemblance between Nidulites and Cyclocrinus on 

 the one hand and certain of the Calcareous Algse (Siphonea verticil- 

 latce) on the other hand ; and it does not seem impossible that these 

 points of resemblance indicate a genuine relationship. If this con- 

 jecture should prove to be well founded, these singular fossils may 

 ultimately find a resting-place in the vegetable kingdom. 



Fig. 74. — Structure of Nidulites ( = Mastopora). 

 a, Tangential section of the wall of Nidulites, show- 

 ing the cup-like plates of the integument transversely 

 divided, so as to give rise to a series of hexagonal 

 cells, enlarged six times ; B, Vertical section of the 

 wall of the same, showing the short hexagonal cups 

 or cells of the outer membrane longitudinally divided, 

 similarly enlarged ; c, A few of the cup-like plates 

 of the wall, viewed internally and showing the cen- 

 tral perforations in their bases, enlarged. From the 

 Silurian rocks of the Island of Oesel. (Original.) 



