628 THE VOYAGE OF H.M.S. CHALLENGER. 



Family XXVI. PYLON ID A, Haeckel, 1881 (PI. 9). 



Pylonida, Haeckel, 1881, Prodromus, p. 463. 



Definition. L arcoidea with regular, incompletely latticed cortical shell, 

 distinguished by two to four or more symmetrically disposed gates or large fissures 

 remaining between one to three latticed dimensive girdles (perpendicular one to another). 

 One, two, or three concentric systems of such girdles (each system with three girdles) 

 may be developed. 



The family Pylonida is the most important and interesting among all the 

 Larcoidea, not only because it is much richer in different and peculiar forms than the 

 other families of this section, but also because it has direct and very complex relations to 

 all the other families of L a r c o i d e a. It is even possible that the Pylonida represent 

 the original ancestral group of the whole section, and that the apparently simpler 

 group of the Larcarida must be derived from the former by retrogressive metamorphosis. 



Till the year 1881 the family Pylonida, which here now exhibits ten genera with 

 eighty-six species, was only represented by one single species, accurately described 

 and extensively illustrated by Johannes Miiller in. 18 58, the well known and widely 

 distributed cosmopolitan Tetrapyle octacantha (Abhandl. d. k. Akad. d. Wiss. Berlin, 

 p. 33, Taf. iii.). A slight modification of it was afterwards described by Ehrenberg as 

 Schizomma quadrilobum (Abhandl. d. k. Akad. d. Wiss. Berlin, 1872, Taf. ii. fig. 12). 

 A more accurate description of it, with a good explanation of its characteristic growth, 

 was given in 1879 by Richard Hertwig in his Organismus der Radiolarieu (pp. 5254, 

 Taf. iv. figs. 7, 8 ; Taf. vi. figs. 2, 5). In my Prodromus (1881, p. 463) I constituted 

 for a large number of allied species, detected in the Challenger collection, the special 

 family Pylonida, and distinguished among it twelve different genera. However, I think 

 it now better to restrict the definition of the family as given in the above definition, 

 and to remove from it a number of genera formerly with it united, as the genera 

 Triopyle and Hexapyle, appertaining to the D i s c o i d e a. 



The characteristic type of all true Pylonida is clearly demonstrated by their peculiar 

 mode of growth, the consequence of which is the imperfect lattice-work of the fenestrated 

 larcoid shell. This remarkable growth is effected by the development of elliptical latticed 

 girdles (or rings), which enclose a quite simple, spherical, subspherical, or lentelliptical 

 primordial shell. The girdles lie in three different planes, perpendicular to one another, 

 and are of different sizes ; each girdle being somewhat larger than the foregoing and 

 somewhat smaller than the following girdle. Between these latticed girdles remain on 

 the surface of the shell large openings or "gates," which are not closed' by network, and it 

 is just the symmetrical disposition and form of these open "gates," separated and enclosed 

 by the fenestrated girdles, which give to the Pylonida their characteristic appearance. 



