20 bulletin: museum of compaeative zoology. 



same specimen has already been published by E. W. Clay pole. ^ Inas- 

 much as this crauium lacks the marginal aud suborbital plates, these 

 have been supplied in the diagram from Xewberry's restoration. The 

 fact that they are shown more in projection than pei'spective imparts a 

 wider and more flattened appearance to the cranium than is strictly nat- 

 ural ; the dorso-laterals are likewise drawn as if flattened out, instead of 

 conforming to the curvature of the body. The outline of the dorso-median 

 has been reduced to scale from a photograph of an exceptionally perfect 

 plate obtained from Dr. William Clark by the British Museum ; its exact 

 position as regards the dorso-laterals has been ascertained from speci- 

 mens in the Museum of Comparative Zoology. Hence the restoration 

 can be considered as such only in the sense that the parts are now 

 brought together in their completeness and proper relationships, and 

 are shown on the same scale. 



The earlier restorations already referred to are subject to the following 

 criticisms. First, the anterior portion of the dorso-median is produced 

 in imagination so as to cover the exposed space behind the occipital 

 region ; secondly, the conditions of overlap and underlap are represented 

 on only one side of the antero-dorso-lateral, instead of on three sides ; 

 thirdly, the postero-dorso-lateral is not shown at all. 



Hitherto the postero-dorso-lateral has never been found in direct 

 association with other plates, and its position has accordingly remained 

 in doubt. It has long been known under j^ewberry's designation of 

 "post-clavicular," and is a plate of not uncommon occurrence in the 

 detached condition. Its triangular form, the markings impressed upon 

 it by overlying plates, and the course of the sensory canal system 

 across it, appeared to the writer'' sufficient evidence for assigning the 

 plate theoretically to the position indicated in the diagram ; and it is 

 therefore interesting to record the discovery of a specimen which estab- 

 lishes the entire correctness of this inference. The new specimen repre- 

 sents the right antero- and postero-dorso-lateral plates of D. terrelli, 

 firmly articulated together, as shown in Plate 2, Fig. 1. It is from 

 the Cleveland Shale,' and was found in the vicinity of Lindale, Ohio, by 

 Mr. Prentis Clark. The inner surface of the plates is alone visible, the 

 external side being embedded in the matrix. The mode of articulation 

 between the two plates is by pegs and sockets, the position of which is 

 fairly constant among the specimens that have been observed. The lar- 



1 Claypole, E. W., The Head of Dinichthys (Amer. GeoL, Vol. X. p. 199), 

 October, 1892. 



2 Amer. Journ. Science, [4], Vol. II. p. 48, July, 1896. 



