Plastron Episternal Structure

Plastron – *The plastron or ventral shield is composed of nine bony pieces, of which eight are paired.

*The anterior pairs of bones are the episternals, the middle pairs are the hyosternals and hyposternals, and the hinder pairs are the xiphisternals. The unpaired piece is called entosternal.

*The precise nature of the bones of the plastron is still unknown.

*Some consider them as a complete structure corresponding with the sternum or breast-bone; others regard them as wholly integumentary; while others, again, hold - what is doubtless the correct opinion - that the plastron is formed partly of bones belonging to the endo-skeleton proper and representing partly the sternum, and partly of integumentary ossifications.

*Both carapace and plastron are covered

*The dorsal surface of the carapace has a median row of five vertebral shields, two lateral rows of costal shields or a marginal row of twenty four or twenty five marginal shields.

*There are six pairs of shields, overlying the plastron. *These shields are gular, humeral, pectoral, abdominal, femoral and anal.

B) Endoskeleton – i) Skull – *The skull is heavy and strong without temporal opening (anapsid condition). * All the skull bones except the lower jaw and the hyoid bone are immovably united together. *The alisphenoid, presphenoid and orbitosphenoid bones are