REPORT ON THE RADIOLARIA. 629 



To understand clearly this peculiar constitution of the Pylonida-shell by a system 

 of alternating girdles, developing one after the other, it is indispensable to pay careful 

 attention to the three different elliptical dimensive planes, which characterise all 

 Larcoidea, and to the three different dimensive axes, which bisect those planes. 

 The girdle which first .develops around the simple primordial shell or central chamber is the 

 transverse girdle, lying in the equatorial plane; then comes, secondly, the lateral girdle, 

 lying in the lateral plane ; and thirdly follows the sagittal girdle, lying in the sagittal 

 or median plane. The three simplest genera of the Pylonida Monozonium, Dizonium, 

 Trizonium represent these three different stages, with one, two, or three girdles. 

 These three genera constitute the first subfamily, Haplozonaria (with one single system 

 of girdles) ; all three girdles lie in the surface of a simple lentelliptical cortical shell. 



From this first subfamily the other two subfamilies of Pylonida must be derived, 

 by repetition of the same characteristic process of growth. In the Diplozonaria a 

 second system of girdles has been developed, constituting a second (outer) cortical 

 shell of lentelliptical form, concentric with the first. Also in this second system 

 the transverse girdle is first developed, secondly the lateral girdle, thirdly the sagittal 

 girdle. The three genera Amphipyle, Tetrapyle (with Octopyle), and Pi/Ionium 

 represent these three different stages of growth. 



Commonly the growth of the Pylonida stops with the completion of the second 

 system ; but sometimes the same process is once repeated and a third system of girdles 

 is formed, constituting a third lentelliptical shell ; in this case also the succession of 

 the three latticed girdles is the same ; firstly the (third) transverse girdle is formed, 

 secondly the (third) lateral girdle, and thirdly the (third) sagittal girdle. Each of 

 these three girdles of the third system encloses concentrically the corresponding girdles 

 of the second and first system. The three corresponding genera of this third subfamily 

 (Triplozonaria) are Amphipylonium, Tetrapylonium, and Pylozonium. But in 

 general this highest number of girdles (nine) is very seldom reached ; commonly the 

 growth of the Pylonida stops with five girdles (Tetrapyle and Octopyle). More than 

 nine girdles I have never observed, though there remains the possibility of the apposition 

 of a fourth system owing to the peculiar imperfect character of the growth itself. 



The central or primordial chamber of the shell, with which in all Pylonida the 

 shell-building commences, is a quite simple, very small fenestrated shell. Commonly 

 one sees on the surface only five to ten small pores (three to four on the diameter). 

 Its form seems to be sometimes spherical, sometimes elongated, ellipsoidal or probably 

 lentelliptical. It may be originally a small Cenolarcus. This simple central 

 chamber, the true " medullary shell " of the small Haplozonaria, is quite different from 

 the medullary shell of the larger Diplozouaria, and particularly of the well-known 

 Tetrapyle. The former observers, J. Miiller as well as R. Hertwig, have described in 

 these forms also the medullary shell as a simple spherical or oblong body. But a careful 



