24 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ventrals may exist iu three different couditious of union ; they may 

 simply overhip, as in Coccosteus ; they may be fused into a single elon- 

 gated piece ; or they may be interlocked with one another. Examples 

 of these three modes of union will now be considered,^ 



Interlocking Median Ventrals. — Two instances have been recorded 

 ■where the median ventral plates of Dinichthys are articulated with one 

 another ; the first was made known by E. W. Claypole in 1893,^ the 

 second by the present \rriter in 1896.^ In both cases the plates oc- 

 curred in the detached condition, and were referred provisionally to 

 the genus Titanichthys. Further investigation has since shown this to 

 have been an erroneous determination, and the only genus that they 

 can be certainly referred to in the present state of our knowledge is 

 Dinichthys. The original of Professor Claypole's figure is preserved in 

 the Museum of the Ohio State University at Columbus. It is a very 

 large and heavy postero-ventro-median, and with it were associated the 

 greater part of the postero-ventro-laterals. The proportions indicate a 

 considerably larger species than either D. terrelli or D. hertzeri, and 

 accordingly the iiame D. ingens * has been suggested for it by A. A. 

 Wright. As a detailed description of these remains is in course of 

 prepai'ation by Professor "Wright, it is sufficient for our purpose merely 

 to cite this as an illustration of a particular mode of union between the 

 median ventrals. 



The other example of articulation or dovetailing is furnished by a speci- 

 men in the Museum of Comparative Zoology, now figui-ed for the first 

 time (Plate 2, Fig. 2). It is broadly lozenge-shaped, and its diagonals 

 measure 20 by 31 cm. The resemblance of this plate to the posterior 

 part of the single element in D. terrelli, already referred to, as figured 

 bv Newberry and Dean, is obvious. Its size, thickness, and markings im- 

 pressed upon it by the paired ventrals, are also in substantial agreement. 

 In these particulars it is seen to be closely allied to D. terrelli ; but on 

 the other hand the articulation with the antero-ventro-median is precisely 

 the same as in D. ingens. The plate in question was collected by Mz-. 

 Terrell, in the Cleveland Shale of Lorain County, Ohio ; but whether 



1 See abstract of a preliminary paper by A. A. Wright, entitled, " New Evi- 

 dence upon the Structure of Diniclitliys " (5th Ann. Eep. Ohio State Acad. Sci., 

 1897, pp. 59, 60). 



2 Report Geol. Survey of Ohio, Vol. VII. p. 611, Plate XL. Fig. 1. 



3 Araer. Journ. Science, [4], Vol. II. p. 47. 



* Should an identity be established between these plates and the mandible 

 described by Claypole as D. hepltri, the latter name is entitled to priority. 



