622 



MOLLUSCOIDEA. 



wards to reach the surface, their walls being considerably thickened 

 in the outer part of their course (fig. 456, b). The earliest forms of 

 Heteropora appear in the Jurassic rocks, and the genus has survived 

 to the present day. The forms included by Busk in Heteroporella 

 are Cretaceous and Tertiary, and the polyzoary is said to differ from 

 that of Heteropora in being discoid and encrusting. 



The family of the Fenestellidce or " Lace-corals " constitutes one 

 of the largest and most important groups of the Palaeozoic Polyzoa, 

 no Secondary or Tertiary types belonging to this family having been 



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m c 



Fig. 465. — Heteropora fiellicidata, Waters (=//. neozelanica, Busk), a recent species ot 

 Heteropora. a, A fragment of the polyzoary of the natural size ; b and c, Portions of the surface 

 of different specimens, enlarged, showing the apertures of the zocecia and cancelli ; d, A portion 

 of the surface of another specimen in which the mouths of the cancelli and some of the zocecia are 

 in places covered by a calcareous pellicle, enlarged. (After Busk, Waters, and the Author.) 



hitherto recognised. In all the members of this family the polyzoary 

 is reticulated, generally fan-shaped or funnel-shaped, and composed 

 of rigid, calcareous, parallel or slightly diverging branches, which may 

 be united by cross-bars or " dissepiments," or may be sinuous and 

 may unite regularly by anastomosis ; the frond coming in both cases 

 to be perforated by symmetrically disposed, sub-quadrate or oval 

 spaces or " fenestrules " (figs. 466 and 467). The zocecia have the 

 form of short, utricular tubes, arranged in two or more series on one 

 side only of the branches, the reverse side being non-poriferous, and 

 being commonly finely striated. When true " dissepiments " are 

 present, they are non-poriferous. The mouths of the zocecia are 



