Y]l Bashford Dean Memorial Volume 



The whole ventral shield (Text-figure 61) is composed of six single plates : two (right 

 and left) antero'ventro-laterals; two (right and left) postero-ventro-laterals; one median- 

 ventral, and one antero-median-ventral. 



Antero-Ventro-Lateral. — This plate (Text-figures 56, 57 and 61 AVL) is 

 large, oblong and flat. It is thickened along the front (IL) and external margins (ex); 

 elsewhere it is relatively thin. The most characteristic point in AVL is a quite long, 

 thick process, forming the extreme angle of its front margin (Sp). Both the front (IL) 

 and the external margins (ex) are concave, the internal (AMV, MV, PVL) on the con- 

 trary forms a strongly arched convex line. 



On the inside (Text-figure 56) the thickened part forms a wall (w) along the front 

 and lorepart of the external margin and strengthens the external process (Sp). This wall 

 goes gradually inward into the inner surface of the plate. On this wall are developed some 

 ridges (rd) and sockets (s^. They are especially clear and deep on the external process. 

 Probably they have served as attachments for ligaments and connecting tissues. The other 

 part of the inner surface of AVL is quite smooth and plane. The overlapping margins are 

 not sharply developed and are difficult to recognise. On the contrary the ossification rays 

 are especially clear. At the same time the growth zone, in the form of concentrically ar- 

 ranged lines, is very remarkable along the inner margin of the plate (Text-figure 56 gz). 



From the outside the plate is still more plane and smooth. It only curves quite 

 steeply downward along the front margin {IL, AMV) and the fore part of the external 

 margin, where some ridges are developed (rd). 



Postero-Ventro-Lateral. — This plate (Text-figures 58 and 61 PVL) is the other 

 large plate of the ventral carapace. It is slightly concave and shows no special thickened 

 parts. The only thing clearly noticeable from the outside is the deep overlapping im- 

 pression on its upper corner (Text-figure 58 AVL) where the hind corner of AVL fits in. 

 On the left plate (looking from underneath) another overlapping margin is to be found. 

 It is situated along the inner margin and was covered by the corresponding right plate 

 (Text-figure 58 PVL). On the inside, this plate is plane and smooth and no clearly 

 overlapping margins are developed. 



Median- Ventral. — This (Text-figures 59 and 61 MV) is a relatively small plate 

 which occupies the central part of the ventral shield and touches all its other five com- 

 ponents. Its outline varies strongly in different species o( Dinichthys, ranging from narrow 

 and long to relatively broad and short. In Text-figure 59 are shown three different M V 

 plates from: (A) Dinichthys terrelli Nwb. (Amer. Mus. No. 7309); (B) Dinichthys (?) 

 intermedius (Buffalo Mus. No. 15608, E2803); and (C) Dinichthys C) curtus (Amer. Mus. 

 No. 7062). 



As we see, this plate is overlapped on every side by the margins of adjacent plates. 

 The part not overlapped forms the inner, thicker division. The upper point of MV is 

 covered by the lower point of AMV; the upper parts of the side margins by the right 

 and left AVL; the lower part of the side margins by the right and left PVL. On the 

 inside, M V is smooth and level, and shows the ossification rays very distinctly. 



