148 NEW YORK STATE MUSEUM 



In plate 13 is shown an exceptionally perfect abdominal plate identi- 

 fiable as the right antero-v.entrolateral of a form closely resembling the type 

 species of Glyptaspis, but distinguished by its more delicate ornamentation, 

 and by a very marked difference in the proportions of length and breadth. 

 As indicated by the trivial title, the plate here named G. abbreviata 

 is very much foreshortened in an anteroposterior direction, more so than in 

 any other genus of Arthrodires with the possible exception of Trachosteus. 

 The general outline is subtriangular, the total length being but 15 cm, and 

 width 16 cm. Although actual specimens are wanting of the corresponding 

 plate in G. verrucosa, we are able to form a tolerably accurate concept 

 of its appearance and general proportions, by means of its impress upon 

 the antero-ventromedian, taken together with the configuration of the 

 postero-ventromedian, and imprints upon it of the postero-ventrolaterals. 

 For all practical purposes therefore, our knowledge of the ventral plates in 

 G. verrucosa is sufficient to permit comparisons to be made with the 

 armoring of other forms ; and in the case of the extremely perfect outlines 

 of G. abbreviata, one can see at a glance that a very different set of 

 proportions obtained in the two species. As regards superficial ornament, 

 the forms are related to each other much in the same way as are B o t h r i o- 

 lepis nitida and B. minor amongst Ostracophores. 



The bone substance of the plate discovered by Dr Clarke is almost 

 entirely denuded, leaving, however, a very clear impression of the external 

 surface. Only the central portion of the plate is tuberculated, the surface 

 sloping away on all sides of this area so as to form a broad, smooth peripheral 

 margin, on which no traces of overlap are to be observed. The details of 

 the ornamentation are shown to rather better advantage in the smaller frag- 

 ment from the Black slate of Lexington, Ky. The specimen here regarded 

 as the type of a new species is from near the base of the Portage beds at 

 Valois, N. Y., and is preserved in the New York State Museum. A some- 

 what larger plate, possibly identical with this, is to be seen in the collection 

 of the Buffalo Society of Natural Sciences, and was obtained by Mr Mixer 

 from the Portage of Sturgeon Point, on the shore of Lake Erie. 



