INVERTEBRATES. 



353 



There is in Pleterosckisma in the proper sense of the word, exteriorly 

 no oral ridge; the part representing it, is concealed from view by 

 the overlapping limbs. The limbs are formed inteiradially into 

 high pyramids with steep sides, of which two of the lateral-walls 

 form the sides of the radial sinus. The pyramids enclose all but 

 the two inner hydrospires, and are, with the exception of these, 

 exposed along the sides of the sinus, but neither their slits nor 

 their folds connect externally with the oral plates. The pyramids 

 produce also important modifications in the form and position of 

 the hydrospires; in Cadaster these organs enter the body nearly at 

 right-angles, and the walls are placed almost parallel to each other ; 

 in Heteroscliisma the hydrospires stand obliquely to the sides of the 

 sinus, and are much wider at the outer side than at the inner. 

 Contrary to the adjoining folds which connect with the slits, these 

 increase in width at the inner end. The hydrospires of Hetero- 

 schisma are arranged closely around the mouth, while those of 

 Cadaster are placed away from the mouth. The differences between 

 these two genera and the allied Phcenoschisma are well seen in the 

 accompanying table : 



Heteroschisma includes the following species : Heteroschisma gracile, 

 the type of the genus; Heteroschisma alternatum, and If. alternatum, 

 var. elongatum, Wachsmuth, 1 all from the lower part of the Ham- 

 ilton group, Devonian. 



iDr.Knapp, of Louisville, Ky., had the kindness to send me for examination i and 

 study, his large collection of Louisville Codasters, consisting of nearly 40 specimens in all 

 stages of growth, and embracing at least two well-marked species. Lyon, in descr Wng 

 his, Cadaster alternate (Owen's Geol. Rep. Vol. III., p. 493), evidently took all Louisville 

 Blastoids containing hydrospire slits to belong to on y one species, and this makes it 

 difficult to ascertain now to which of- the two forms the name sh W4 b appUeg- flm 

 generic figure, on PL V., Fig. 3, represents undoubtedly the form which afterwards was 

 described by Shumard as Uodaster pyramidatus (Acad. 8ci.,bt. Louis. Vol._ 1., Ho. 4, 

 Lyon's figure 3 b gives a small specimen of the same species; his figure 3 a ^owever. is 

 not only an entirely different species, but a HeteroscJusma. I* ^* PPS ^niiis 

 form represented by the first figure should receive Lyon s name, but, his would do jus- 

 tice to Shumard, the more as Lyon's descriptions are fitted for neither the one form nor 

 the other. It must be further considered that the latter form, whio r le fl o* chi 

 gracile, has small orals overlapped by the limbs, and large pyramids between tl 

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