134 Prof. F. J. Bell on the Laganidse. 



separation " of L. Peroniij or, as did Gray, to divide the 

 species of the genus into three groups, one with five, one 

 with four apical, and one with four extrapetaloid genital 

 pores. 



No capable naturalist has ever yet given a generic position 

 on the strength of the character now under discussion, or so 

 acted as to render necessary the protestation that u this fea- 

 ture alone cannot be of any generic value." 



On the other hand, it is to be borne in mind that the change 

 that has taken place in the position of the genital pores is, as 

 Loven has shown, one that affects rather the ducts than the 

 so-called genital plates ; and that being so, it is clear that we 

 have to do here with an arrangement which, in two organisms 

 of essentially the same structure, might, under similar condi- 

 tions, independently appear : or, in other words, we have a 



g 



ood and sufficient reason for not using the position of the 

 genital orifices as the bases for generic coupes. So far as this 

 particular character is concerned, we may safely say that it 

 affords no evidence in favour of a closer alliance between 

 L. Putnami and L. Peronii than between either of these and 

 some other members of the genus. 



We may now pass to the second distinguishing character, or, 

 rather, to that which has been used as such. In recognizing 

 the subgeneric distinctness of Peronella, Prof. Alex. Agassiz 

 states that he bases "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 halfway 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." 



1 shall now endeavour to show that these differences in 

 structure are really due to differences in age ; but, for the 

 purposes of the investigation, I may limit myself to that part 

 which lends itself most easily to accurate and detailed mea- 

 surements. 



A specimen of P. decag 'onalis , determined by Prof. Alex. 

 Agassiz, and coming from St. 212 (II. M.S. i Challenger ? ), 

 with four genital pores, has a radial measurement of 223 mm. ; 

 and the partitions extend inwards 7*3 mm. from the edge, or 

 rather less than one third of the distance. 



A specimen, bearing the same name, from St. 219, with 

 five genital pores, with a radial length of 22 mm., has the 

 partition 85 mm. from the edge, or about two fifths, but 

 still considerably less than one half. 



Similar results are obtained by the measurements of other 







