i86 



FISHES 



the primitive rhomboidal shape of the dermal denticle is retained ; 

 in others a rounded or cycloid scale supplants the earlier rhombic 

 type. Within the limits of the same group {e.g. Crossopterygii) 

 there are examples of the independent evolution of a cycloid from 

 a pre-existing rhombic squamation ; and with the introduction of 

 the cycloid type an overlapping or imbricated disposition of the 

 scales always takes the place of the marginal articulation of the 

 rhombic type. 



As to the causes which may have determined the shape and 

 mutual relations of scales interesting suggestions have been made.-^ 

 Scales bear a segmental relation to the subjacent muscle-segments 

 or myotomes, sometimes being disposed in oblique transverse rows 



Set.- 



Fig. 1 1 . — Development 

 of a scale in L&]n- 

 dostexis osseus x 330. 

 b.p. Basal plate, with 

 included bone cells, 

 at first distinct from 

 the spine ; e, enamel ; 

 e.o, enamel organ ; 

 epj epidermis, with 

 large gland cells ; 

 p, dermic papilla 

 which forms the ves- 

 tigial spine ; Sd, 

 scleroblasts. (From 

 Klaatsoh. ) 



coinciding with the latter, or the rows may be so far increased as 

 to be multiples of the myotomes. From mechanical considerations 

 depending on the sigmoid shape and interdigitating relations of 

 the myotomes and their separating fibrous septa or myocommata, 

 and the attachment of the myocommata to the dermis, the con- 

 traction of the myotomes during the lateral flexions of the trunk 

 in swimming has a tendency to wrinkle the skin into definitely 

 circumscribed rhombic areas, thus determining the shape, limits, 

 and disposition of the scales which are developed in those areas. 

 The rhombic was probably the primitive shape of scales, and is 

 certainly characteristic of the palaeontologically older types of 

 scaly Fishes. Generally the rhombic condition is associated with 

 a peg-and-socket articulation between the upper and lower margins 

 of adjacent scales. But a rhombic squamation is not without 



' Ryder, Proc. Acad. Nat. Sci. Philadelphia, 1892, p. 219 ; Smith "Woodward, 

 Nat. Sci. iii. 1893, p. 448. 



