INSTITUTE OF SCIENCE, PHILADELPHIA. 537 



The only parts of the septum in Ciispidaria and Cetoconcha which can 

 really be said to be derived from the gills, as such.are the margins of the foram- 

 ina, including the foramina themselves, as Grobben's sections show clearly. 

 To regard as derived from the gills the muscular fascia which cross and curve 

 over the septum seems to the writer illogical and not accurate. The ques- 

 tions involved, however, can only be satisfactorily settled by studies of the 

 development in younger stages. 



The ordinary retractors of the siphons remain represented in some species 

 by a few short fibres, as shown by Grobben in C. aispidata, but these are only 

 feebly functional, and do not sinuate the pallial line. The strong septal mus- 

 cles pull directly on the siphonal septum, and in Dermatomya,2,=, I have shown 

 elsewhere, a sort of transfer of muscular fibre from the pallial to the septal area 

 seems to be going on behind the siphons. Those species of Poromya which 

 have strong siphonal retractors have few muscular fibres in the septum and 

 vice versa. 



Under the facts as now known there seems to be no warrant, as Grobben 

 observes, for assigning an ordinal weight to the group called by Pelseneer 

 Septibranchia. The fact that the gill-structure is more nearly foliobranchiate 

 than reticulate is of more importance, though Pelseneer has not recognized it 

 in his classification. 



Order TELEODESMACEA. 



A. Pantodonta. 

 Laterals exceeding two in any one group. 



■ Family ALLODESMID^. 



Shell rounded ; valves equal, free, closed, with feeble concentric sculpture ; 

 area linear, amphidetic ; ligament subexternal, parivincular, opisthodetic ; ad- 

 ductor scars subequal, pedal scars above and distinct from the adductors ; pal- 

 lial line entire ; hinge with one or two lateral laminse on each side of the beak, 

 the posterior below the ligament, received into corresponding grooves on the 

 right valve ; cardinal teeth radially grooved, one or two in each valve, those 

 in the right valve stronger. 



Silurian. 

 Ex. Orthodontiscus, Allodesma. 



This family, as suggested by Neumayr, probably exemplifies the first step 

 in the development of the Teleodesmacean hinge. But it must be admitted 

 that its amphidetic though linear area, the occasional multiplication to three 

 of the lateral laminae, and the subligamentary location of the hinder laminae, 

 are very reminiscent of the prevalent Silurian schizodont type, and the family 

 can be admitted to the Teleodesmacea only as a probable ancestor, rather than 

 a perfectly developed type of the modern assemblage. 



