338 PROCEEDINGS OP THE ACADEMY OP 



title of tramitionary, by which term I mean that these forms may be regarded 

 as just such aberrant varieties of that genus as we might expect to find con- 

 ducting to the Genera Nitzschia and Synedra, which, on this continent, appear 

 to have followed the genus Surirella at a long interval of time. They exhibit, 

 moreover, such very unusual variation, both as to size, configuration and de- 

 finition of distinctive character ; such want of generic fixity, to so express 

 it, as might be supposed likely to mark the incoming of new Genera. 



Although there may exist reasonable ground for doubt as to whether the 

 lines which separate two of these genera (Surirella and Amphipleura,) or some 

 of their species, are not founded on structural degradation, rather than on 

 true and definite characteristics, the evidence I have to offer is, as yet, not 

 sufficiently digested to make it in any respect conclusive of the matter. It 

 will be briefly stated hereafter in the consideration of species. In the mean- 

 time I prefer to consider these eccentric members of the Surirellae as illustra- 

 tions of the " Comprehensive " type of Dana, (Synthetic of Agassiz,) which he 

 explains to be " one which embraces, along with the characteristics of the 

 group to which it belongs, others of another group, and usually at its first ap- 

 pearance this group is not in existence." "It is in part" (he adds,) an "interme- 

 diate type between two groups, although never occupying the middle point, 

 as it always belongs fundamentally to one of the two, while partaking of some 

 of the characteristics of the other. This comprehensive type, moreover, be- 

 comes extinct with the progress of the system of life, while the types which 

 it foreshadowed, or partly comprehended in itself, are long afterwards perpe- 

 tuated," and such, to all appearance, has been the fate of these curious in- 

 termediate links between the genera in the present case.* 



In conformity with this definition of the comprehensive type, a careful 

 comparison of these Nitzschoid and Synedroid varieties has convinced me 

 that all of the doubtful and imperfectly defined species really belong to 

 Surirella ; even the necessity of creating a Subgenus for their accommodation 

 being obviated by the clearly marked progressive series of intermediate forms 

 connecting the normal type of Surirella with the most aberrant varieties in 

 the Saco mud. 



The species first to be described I have named after the late Prof. Bailey. 



I. Sukirella Bailbyi, n. sp. F. V. Oblong linear, with rounded extremities. 

 V. linear lanceolate, slightly attenuated near the central portion, with the 

 rounded apices more or less produced. Alas conspicuous. Canaliculi well 

 marked ; striae usually distinct up to the well defined central line. Owing to 

 the great variation both in the number and fineness of the striae and canal- 

 iculi, I have not attempted to estimate their average. Habitat : in the soft 

 mud about and under the roots of the grass at the north end of the pond. 

 The silicious valves occur, as is the case with nearly all the ^succeeding spe- 

 cies, in greatest abundance at from two to six inches below the surface. 

 Fig. 1, a, and b. 



This form is rare in the gathering. It is an exceedingly elegant little spe- 

 cies. The smaller valves of what I believe to be the same form, bear a super- 

 ficial resemblance, in outline and punctulation, to Nitzschia the alae and 

 canaliculi becoming so much dwarfed as to be nearly invisible. A careful 

 manipulation of the valve dispels this apparent similarity, reducing both lines 

 of seeming puncta to the same plane, and also defining their structural 

 value as alas. 



Although no doubt can exist as to the claims of this species to rank as a 

 Surirella, I have preferred to first describe it before proceeding to the forma 

 hereafter to be noticed, which depart widely and eccentrically from the type 

 of the genus in question, for the reason that some of its varieties serve as an 

 introduction to these latter, whose anomalies they, in a measure, explain. 



* Manual of Geology, page 395. 



[Deo. 



