628 



MOLLUSCOIDEA. 



The genus Acanthocladia is Carboniferous and Permian, and possesses 

 a polyzoary which is bilaterally branched in a single plane, and which is 

 celluliferous on one side only, the reverse side being solid and striated. 

 The zocecia are placed on the main stem and branches, and are multi- 

 serial. The Carboniferous genus Pinnatopora (the Glanconome of many 

 authors) resembles the preceding in general form (fig. 471, b and b'\ but 

 the cells are biserial, and the stems have a more or less well-developed 

 keel. The Carboniferous genus Septopora resembles Pinnatopora gen- 

 erally, but becomes fenestrated by the union 

 of the lateral branches of the polyzoary ; while 

 the same thing occurs in the Permian genus 

 Synocladia. In this last genus the lateral 

 branches are directed obliquely upwards, and 

 carry two rows of pores each, while the main 

 stems carry from three to five rows of pores 

 separated by a median keel. 



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A 



The families of the Ptilodictyonidce, Stic- 

 toporidce, and Cystodictyonidce. may be briefly 

 considered together, since they contain 

 Palaeozoic Polyzoa which are superficially 

 very similar to one another, and which are 

 in many respects really allied, though they 

 differ in the internal structure of the poly- 

 zoary. Owing, however, to the fact that 

 various types have been hitherto insuffi- 

 ciently examined by means of microscopic 

 sections, it is not always possible to separ- 

 ate these families accurately, or to refer a 

 particular form to one or other of them. 

 The family of the Ptilodictyonidce, more 

 especially, cannot at present be precisely 

 defined, since the structure of the type- 

 species of the genus Ptilodictya — viz., the 

 P. lanceolata, Goldf. sp. of the Silurian 

 rocks — has not yet been thoroughly in- 

 vestigated. Judging, however, from ex- 

 ternal characters alone, it would seem 

 probable that the Devonian Polyzoan de- 

 scribed by the writer under the name of 

 Heterodictya gigantea is congeneric with 

 Ptilodictya /anceo/ata, as understood by 

 Lonsdale ; and we may therefore take the 

 former as exhibiting the essential characters of the genus Ptilodictya. 

 The polyzoary in Ptilodictya (Heterodictya) gigantea has the form 

 of a flattened, unbranched, two-edged frond, which reaches several 

 inches in length and an inch or more in width (fig. 472), with a 

 thickness in the centre of about two lines. The polyzoary consists 



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Fig. 472. — A specimen of 

 Ptilodictya {Heterodictya) gi- 

 gantea, from the Devonian 

 rocks of Canada, of the natural 

 size. The polyzoary is split in 

 half along its median plane, and 

 shows in places portions of the 

 striated calcareous membrane 

 produced by the coalescence of 

 the two layers of cells compos- 

 ing the colony. (Original.) 



