1. Placoid（denticles)
--toothlike, being made of dentin with a pointed backward
projection of enamel, as in sharks and rays
-- consists of a flattened rectangular basal plate in the upper
part of the dermis.
--has a cup or cone of dentine with a pulp cavity richly
supplied with blood vessels.

2. Cycloid and ctenoid scales are overlapping,flexible bony ridges.
-Cycloid scales have a smooth posterior margin lacking ctenii.
- smooth rim
ctenoid
-Ctenoid scales have a variously developed spiny posterior margin
- comb-like rim
-three types:(i)crenate, with simple indentations in the margin
(ii)spinoid, with developed spines that are continuous with the main body of the scales
(iii)ctenoid,with ctenii formed as separate ossifications distinct from the main body of the scales.

4. -placoid scales are characteristic of
the Chondrichthyes.
-cycloid or ctenoid scales are
present in the Teleostei.
--Cycloid and ctenoid scales consist of two
main regions, a surface 'bony' layer, composed
of an organic framework impregnated largely
with calcium based salts, and a deeper fibrous
layer composed mainly of collagen.

5. placoid scales do not increase in size with growth,but as a fish
with cycloid or ctenoid scales grows, its scales also grow.
placoid scales are both epidermal and dermal portions.But cycloid and ctenoid are almostly dermal.
--the outer player of the placoid scale is hard,enamel-like vitrodentine,derived from ectoerm.
--cycloid and ctenoid scales are more flexible.