520 PEUOXELLA. 



(LAGANUM. ) Peronella. 



Peronella Gray, is;,;,. Cat. Rec. Ech. 



A.gassiz had already hinted at the probable generic separation of L. 

 Peronii from Laganum : this has boon adopted without further proof by 

 Gray, Desor, Michelin, and Ilupe. who have independently established a 

 distinct genus, based upon the peculiar position of the genital opening, far 



away from the abactinal system, in the interambulaera ; hut as we have a 

 true Laganum in which the genital openings (L. Putnami) have the same 

 extrapetaloid position, this feature alone cannot be of any generic value. I 

 am inclined to adopt this group as a .subgenus, but basing the distinction 

 entirely upon the internal structure of the test in Peronella. The partitions 

 forming the connecting walls between the upper and lower floor ramify 

 somewhat as they do in Scutella and Arachnoides, and extend more than 

 hall-way to the centre of the test from the edge, instead of forming a narrow 

 belt of three or four concentric simple walls near the edge. Four genital 

 openings. In young specimens of P. decagonalis the partitions are already 

 well developed. 



Peronella decagonalis 



! Sculella decagonalis Less, 1*27. in I!r . Diet. Sc. X;it. Scut., p. 229. 

 Pi ron< lla rfi i agonalii A. A.G . 1872, IU\. Ech . Pt. I. p. l I*. 



Pi XIII'. f. s i, ; Pi XXXVII. f. .;. 



This is the largest and mosl graceful of the species of Laganidae. Its 

 origin having been till lately unknown, it has been described independently 



several times. Its external features are sufficiently striking to have led 

 Agassiz to suggest its separation from Laganum ; this suggestion was carried 

 out first by Gray and subsequently by Desor. Michelin, Dujardin, and Hupe 

 later described the species as new. and formed a genus for its reception. 

 The test is very much Battened ; the edges are scarcely swollen, with a 

 slight depression between the edge and central portion of the test, as in 

 Laganidae generally ; the central part of the test rises somewhat abruptly 

 at the extremity of the petals, and is regularly arched, hut only lies to a 

 moderate height even in the largest specimens. The outline is regularly 

 decagonal ; the test is truncated by ten sides, the longer sides corresponding 

 to the interambulacral; and the shorter to the ambulacral area;.. 



