30 PALEONTOLOGY. 



along with the corals, if only on account of the difficulty he, 

 in many cases, experiences of determining to which class 

 of Polypi his specimens belong. M. d'Orbigny, who has 

 devoted much attention to this class, assigns more importance 

 to the form than to the grouping of the cells. These are 

 marked by "pores" and "pits," the variation of which helps in 

 many instances to the appreciation of minor natural groups ; 

 but the members of such groups differ greatly in the 

 general form of the polypary, which may be an encrusting 

 sheet, or may rise in plates or in branching stems. The 

 number of extinct species must be great, since the Bryozoa of 

 the chalk, which alone have been carefully examined, amount 

 to 213 ; while only two species are known from the trias, none 

 at all from the lias, and only five from the upper oolites, so 

 rich in corals and sponges. In the " Cours Elementaire" of 

 d'Orbigny the fossil Bryozoa are stated to amount to 1676, 

 distributed in 85 genera. 



Of the 19 or 20 palaeozoic genera, none extend into the 

 secondary strata; but of the 18 oolitic genera, EntalopJwra 

 and Defrancia range onwards to the tertiaries; and Alecto, 

 Idmonea, and Eschara still survive. The oldest known fossil, 

 Oldhamia (fig. 3, 2), has been supposed to be a Bryozoon, as has 

 been likewise the Graptolites (fig. 3, 3). The most common 

 palaeozoic form is Fenestrella (fig. 3, 1 1 ), resembling the recent 

 " lace-coral" ; there are 35 species, ranging from the Lower 

 Silurian to the Permian. One of its modifications resembles 

 a feather (Ptilopora, fig. 3, 10), and is found in the carbonifer- 

 ous limestone. Another, more remarkable, has a spiral axis 

 (Archimedvpora, fig. 3, 9), and occurs in the same formation in 

 Kentucky. One of the oldest genera is Ptilodictya (fig. 3, 8), 

 of which seven species are found in the Lower Silurian forma- 

 tions. The slabs of Silurian limestone obtained at Dudley are 

 covered with myriads of small and delicate fossils, including 

 many Bryozoa. Some of these are spread like a film over 



J 



