24 



Fishery Bulletin 89(1), 1991 



calcifications within the basalia have the shape of a 

 Maltese Cross (White 1937) (Figs. IB, 6). This "car- 

 charinoid" pattern is characteristic of the families 

 Triakidae, Sphyrnidae, and Carcharhinidae (Applegate 

 1967). Above the centrum is a neural arch. In the center 

 of the centrum is a hole, marking the position of the 



Figure 8 



Microradiograph of sagittal section of Galeorhinus galeus 

 vertebra from Figure 7. The more calcified zones (rings and 

 cone in general) appear white, and the less calcified zones 

 (opaque zones and intermedialia in general) appear dark. 



primitive notochord, which we adopted as the focus of 

 the vertebra. Within each cone, the focus is surrounded 

 by a series of concentric rings which are read through 

 techniques using the whole centra. 



The inside of each cone is lined by a perichondrium, 

 which consists of a fibrous layer covering a germinative 

 layer of chondroblast cells (Fig. 9). During growth 

 phases, the chondroblasts differentiate into chon- 

 drocytes to form the mature cartilage, a densely 

 cellular tissue consisting of rounded cells embedded in 

 their secreted organic matrix. The body of the vertebra 

 forming the intermedialia is also invested by a peri- 

 chondrium. In sagittal section the differences between 

 the two regions can be observed: the cells of the cone 

 are smaller and embedded in a more abundant matrix 

 than those of the intermedialia (Fig. 10). Mineraliza- 

 tion occurs throughout the matrix and both regions 

 present an alternate pattern of more and less mineral- 

 ized zones, corresponding to the concentric rings that 

 can be seen inside the cone. 



The properties of the narrow and wide zones, which 

 occur in an alternating sequence, were defined by com- 

 paring microradiographs and direct observations with 

 transmitted and reflected light of sections of the same 

 vertebra. In this species the narrow zone, which we 

 define as a ring, is optically translucent and appears 

 white on the radiograph, being opaque to the X-ray 

 beam, and therefore more calcified. The wide zone, 

 defined here as a growth zone, is optically opaque and 

 appears dark on the radiograph, being semi-trans- 

 parent to the X-ray beam, and therefore less calcified 

 (Figs. 7, 8). 



Figure 9 



Sagittal section of Galeorhinus galeus 

 vertebra showing part of the cone and 

 intermedialia. Starting from external 

 side: a = perichondrium; b = ring 

 crossing cone; c = ring crossing inter- 

 medialia. (Harris's haematoxylin, 

 100 x) 



