REPORT ON THE RADIOLARIA. 481 



Family XXI. PORODISCIDA, Haeckel (Pis. 41-47). 



Porodiscida, Haeckel, 1881, Prodromus, p. 459. . 



Trematodiscidu et Discospirida, Haeckel, 1862, Monogr. d. EadioL, pp. 485, 491, 513. 



Calodictya, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 53 (partim). 



Definition. D i s c o i d e a without phacoid shell, with flat discoidal' shell, in which 

 a simple spherical central chamber is surrounded by concentric chambered rings (each 

 ring divided by radial beams into imperfect chambers). Surface of the disk on the 

 two flat sides covered by a porous sieve -plate. 



The family Porodiscida is by far the largest and richest in different and 

 common forms among all Discoidea; already in my Monograph (1862) nine 

 genera and twenty -eight species have been enumerated. Their number is here increased 

 to more than thirty genera and two hundred species. Many of these species appertain 

 to the most common and widely distributed SPUMELLARIA, both living and fossil. But 

 the study of their structure is not easy, and requires (as in the foregoing Coccodiscida) 

 not only careful examination of the facial views of the disk, but also of the marginal 

 view and of slides and sections through different planes. 



In my Monograph (1862, pp. 485, 491, 513) I had constituted for these 

 Discoidea two different families, the Trematodiscida and Discospirida ; but the 

 comparative study of a far greater number of different types in the Challenger collection 

 has since convinced me that those two families are but little different, and united 

 by transitional forms within one and the same genus, so that they must be united as 

 Porodiscida. Of the group, which Ehrenberg formerly had called " Calodictya," many 

 genera appertain to the Porodiscida, whilst many others' are true Spongodiscida. 



The Porodiscida represent the first and the most important family of the 

 Cyclodiscaria, or of those Discoidea which are devoid of the peculiar extra- 

 capsular lenticular " phacoid shell," characteristic of the three preceding families (united 

 therefore as P h a c o d i s c a r i a). Probably all Cyclodiscaria can be derived 

 from Archidiscus, from a morphological as well as a phylogenetic point of view. 

 Archidiscus seems to be the common ancestral form not only of the Porodiscida, but 

 also of the nearly allied Pylodiscida and Spongodiscida. This important Archidiscus 

 (PI. 48, figs. 911) is a small lenticular circular disk, in which a simple latticed 

 spherical central chamber is surrounded by one single concentric ring, connected with 

 it by a variable number of radial beams in the equatorial plane. From this typical 

 Archidiscus, as from their " architype," all other Cyclodiscaria may be derived ; 

 the Porodiscida by regular apposition of new concentric chambered rings on the margin, 

 the Spongodiscida by irregular apposition of a spongy framework, the Pylodiscida by a 

 peculiar interrupted, concentric, triradial growth, three radial arm-chambers alternating 

 with three open gates or holes, so that already the first chambered ring is not complete. 



(ZOOL. CHALL. EXP. PART XL. 1885.) Rr 61 



