460 Dr. Wallich on the RMzojjods. 



like manner rectangular. Again, in pi. xvi. fig. 22, repre- 

 senting Diffiugia glolndosa^ we have a test described as being 

 " composed of rectangular plates, together with a few ^ dia- 

 toms." This is a most interesting specimen, inasmuch as 

 the entire external surface of the test (an oblate spheroid) is 

 closely studded over with rectangular elongated plates, a 

 broadly ovate valve of a diatom (apparently a Cocconeis) being 

 faintly visible as adherent to the exterior of the plates, though 

 so hyaline as not to obscure the plates on the part of the test 

 covered by it. This form must be regarded not as a tran- 

 sitional variety between Diffiugia symmetrica and those forms 

 of test made up of an admixture of metamorphic bodies (such 

 as were stated to be present on the test figured in Prof. 

 Leidy's plate x. fig. 26, supra), but as a form which is essen- 

 tially a second ''representative" of the so-called genus 

 Quadrula, and is actually inserted in a plate devoted exclu- 

 sively to six forms of Diffiugia, viz. glohulosa, hhostoma, 

 arcula, urceolata, cratera, and pyriformis. To deny this 

 because the figure of one is " pyriform compressed,", whereas 

 the other is a compressed spheroid, or because the rectangular 

 plates in one are true squares, whereas in the others they are 

 parallelograms, or even because in Diffiugia symmetrica there 

 is a close approach to symmetrical arrangement of the square 

 plates, whereas in the other the elongated rectangular plates 

 are not so symmetrically arranged, would be too absurd, when 

 it is borne in mind that we are discussing the predominating 

 characteristic so conspicuously manifest throughout the whole 

 of the vast series of testaceous E-hizopods, namely an infinite 

 tendency to variation — a tendency which even Prof. Leidy 

 himself admits while making a new genus out of Diffiugia 

 symmetrica. The only matter for surprise is that so keen an 

 observer as Prof. Leidy should not, at the first glance, have 

 detected the intimate connexion just indicated between 

 " Quadrula " and the specimen of Diffiugia glohulosa to which 

 I refer. Curiously enough. Prof. Leidy figures, side by side 

 with this specimen of D. glohulosa, a second specimen of the 

 same form (fig. 21), almost identical in size and quite identical 

 in figure, in which there are no rectangular plates or disks of 

 any kind, but the entire test is covered with diatoms as large 

 as the Cocconeis in fig. 22. The diatoms in fig. 21 are, how- 

 ever, pointed ellipses, and they are as yet metamorphosed 

 only to the extent of obliterating their generally very coarse 

 striation or cellulation, another significant circumstance in 



• Only one diatom-yalve is visible on the test in fig. 22. There must 

 therefore be some inaccui'acy in the explanator}' note. 



