The Structure of Dimchthys 



173 



AVL 



The postero-ventro'lateral plate shown 

 from the outside. 



Text-figure 59. 

 Three different median-ventral plates. 



A, Dmiclithvs terrelh Nwb. (American Museum No. 7304) shown 



from outside; B, Dimchthys (7) mtermedius Nwb. (Buffalo Museum 



No. 15608-E2803J from outside. C, Dmchthys (') cunus Nwb. 



(Ameri can Museum No.' 7062) from inside. 



Antero-Median-Ventral. — This plate (Text-figures 60 and 61 AMV) fills up 

 the cleft between the two antero-ventro-laterals and forms the median strongly curved 

 part of the ventral shield's front margin. It is a small plate with an arched front margin 

 and concave side margins. Broadest in front, it becomes narrow backward, where the side 

 margins run together in a roundish point. On the inside, its fore part is quite deeply 

 impressed; thus the front margin forms a semicircular wall. A T-shaped thickening 

 consolidates the fore part and the median line of the plate (Text-figure 60B t). 



On the outside AMV is smooth. Its front margin is curved downward, correspond- 

 ing to the front margin of AVL. AMV is slightly overlapped by AVL along the side 

 margin, and its posterior point strongly overlaps the anterior part of M V. 



In a paper published in 1896, Dean described a ventral shield of Dimchthys terrelli 

 Nwb., noting that in this form the M V and AMV instead of forming two plates, formed 

 only one. In that same year, Eastman pointed out the improbability of this fact. He noted 

 that in all other known forms of Dimchthys, as in all other Arthrodira, these plates are 

 developed as two isolated plates. Later Wright (1897) and Hussakof (1906) mentioned 

 this question. Hussakof conjectured that in primitive forms of Dinichthys AMV and M V 

 were isolated, but that in the more specialized forms these two plates fused. 



