REPOET ON THE RADIOLAEIA. 629 



To uuderstaud clearly tins jjeculiar eoustitutiou of the Pyloniila-shell ])y a system 

 of alternating girdles, develo^jiug one after the other, it is indispensable to pay careful 

 attention ' to the three ditiereut elliptical dimensive planes, which characterise all 

 L a r c i d e a, 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 e(|uatorial }ilane ; then comes, secondly, the lateral girdle, 

 lying in the lateral plane ; and thirdl}* follows the sagittal girdle, lying in the sagittal 

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

 Trizonium — represent these three difierent stages, with one, two, or three giixUes. 

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

 of girdles) ; all three gmlles lie in the surface of a simple leutelliptical 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 leutelliptical form, ctoncentric 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 Amphipijle , Tetrapyle (with Octopyle), and Pi/Ioiiium 

 represent these three diff"erent stages of growth. 



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

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

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

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

 secondly the (third) lateral gii'dle, 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 subf;iniily 

 (Triplozonaria) are Amphijyyloniiim, Tetrapylonimn, and Pylozoninm. But in 

 general this highest nunilier 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 chamljer 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 difierent from 

 the medullary shell of the larger Diplozonaria, and partici;larly of the well-known 

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

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



