Buccopharyngo-branchial denticles of Phlebolepis elegans Pander (Thelodonti, Agnatha)

Denticles and denticulate platelets belonging to internal structures of Phlebolepis elegans Pander, along with two new types of scales, are described for the first time. Large fragments taken from seven articulated specimens of P. elegans were dissolved in acetic acid. The denticles were picked from the resulting residues where they were found adhering to the visceral surfaces of scales in samples taken from the anterior part of the body. The overall shapes of the tiny denticles vary from rhombic to elongate, and the sculpture on their surface consists of longitudinal ridges of different lengths and/or tubercles. Denticles are also covered with an ultrasculpture of longitudinal striations. The shape of each denticle varies depending on where it was situated in the body. Rhombic or roundish denticles were found together with rostral and short cephalo-pectoral scales, the latter often pierced by pore canals of the lateral line system; strongly elongate denticles were associated with long lateral scales. These newly described denticles covered the buccopharyngobranchial region of P. elegans. Denticulate platelets also came from the branchial region. Two sets of scales in a slightly crescent-shaped arrangement are bordered on one side by very narrow and high scales. These crescent-shaped sets bordered openings, which are here interpreted as branchial openings. The openings were situated between long lateral scales, and each successive opening was placed slightly ventral to its more anterior neighbor. These internal denticles reinforce the idea that such structures are general features of thelodonts, and not specializations of restricted groups of thelodonts.