376 THE VOYAGE OF H.M.S. CHALLENGEE. 



TJie Medullary Shell is always double in this family, composed of two small 

 concentric shells lying in the middle of the central capsule. They are either spherical 

 or lenticular, compressed towards both poles of the main axis ; sometimes the inner is 

 spherical, the outer lenticular. From the equatorial circumference of the latter starts a 

 number of radial beams, which perforate the central capsule, and are inserted into the 

 equatorial constriction of the cortical shell. Commonly all radial beams lie in the 

 equatorial plane, sometimes also a part of them on both sides of it. 



The Cortical Shell is constantly composed of four completely latticed chambers, 

 which are only separated by the three annular constrictions. In many species all four 

 chambers have the same form and size (PL 40, fig. 3), but in the greater number of 

 species both proximal (or central) chambers are different in size, shape, or structure from 

 both distal (or polar) chambers. The cortical shell is either simple or double, composed 

 of two concentric four-jointed shells ; rarely it is triple or multiple, composed of three or 

 more shells fitting one inside the other. Very often the outer cortical shell is incomplete, 

 and only developed around the two proximal chambers of the complete inner cortical 

 shell, both distal chambers of the latter remaining simple. 



On both poles of the main axis often are developed solid spines (Panicium, PL 40, 

 fig. 4) or hollow fenestrated tubes (Panarium, PL 40, fig. 9), as in the foregoing families. 

 Besides this, the surface of the cortical shell may be armed with spines. 



The Central Capsule of the Panartida is always cylindrical, on both poles hemi- 

 spherical ; . commonly it is more or less distinctly four-jointed, with three ring-like 

 constrictions corresponding to those of the enclosing cortical shell. From the inner 

 surface of the latter it is separated by a thinner or thicker jelly-mantle. 



Synopsis of the Genera of Panartida, 



On both poles of the main axis neither ( Cortical shell simple, . . . 163. Panartits. 



peculiar polar spines nor fenestrated < 

 tubes. ( Cortical shell double or triple, . .164. Peripanartus. 



On both poles -bf the main axis a lar ? e j Cortieal she11 sim P le ' 1G5 ' 



apical spine or a group of polar spines, j ^^ ^ ^^ Qr ^^ ^ Peripaniewm. 



On both poles of the main axis a conical j Cortical she11 sim l )le ' 1G7 ' 



or cylindrical fenestrated tube. )r>t-iiiiiii * i i^on- 



( Cortical shell double or triple, . . 168. Pertpanarutm. 



Genus 163. Panartus, 1 n. gen. 



Definition. Panar t i d a with simple cortical shell and double medullary shell, 

 without polar spines or tubes on both poles of the main axis. 



1 l'anarlus = quite breail-like ; 7fa.iia.yTos. 



