352 THE OSTRACODERMS. 



dentine, with an underlying stratum of Haversian, or other, canals. The lamellae 

 are always parallel to, or concentric with, the walls of the cancellae, or with those 

 of the larger spaces they enclose. Openings through the inner layer serve for the 

 passage of blood-vessels and other tissues into the cancellae, and hence to the canals 

 of the outer layer. 



The outer surface is denser and harder than the rest, and may be without 

 distinct dentinal tubules or lamellas, thus forming a thin enamel, or ganoin layer. 

 It is often divided into small polygonal areas, and is variously ornamented with 

 tubercles or ridges. 



The most primitive lacunas are unipolar, or bipolar, and arise as dilata- 

 tions of the inner ends of primitive pore canals (Pteraspis) ; or a linear series of 

 lacunas may be formed from local enlargements of a single canal (Tremataspis). 

 The more highly developed, or typical bone lacunas, arise from the unipolar 

 lacunae through the formation of secondary lateral canals, or canaliculi. The 

 primitive lacunae are located mainly, and primarily, in the deeper lamellas: that is, 

 in the axial portions of the trabeculae, and in the partitions separating the cancellae 

 and larger canals. As the lacunae develop in size and complexity, they lose their 

 original arrangement and their relation to pore canals, taking up their position 

 between the lamellae, with their long axes parallel with one another and with the 

 plane of the lamellae, those in one layer often standing at right angles to those in 

 the adjacent layers. The lacunae of the same and of the adjacent layers are then 

 united by many branching canaliculi. As the latter increase in numbers, they 

 form an anastomosing network that takes the place of the primitive unbranched and 

 parallel pore canals. The primitive condition of the pore canals is, however, 

 largely retained in the outermost layers of the shell, forming the dentine layers 

 characteristic of the surface ridges, spines, and tubercles. 



The general trend of development in the exoskeleton of the ostracoderms 



is as follows: 



1. The lacunas become parallel with the lamellae, instead of with the pore 

 canals. 



2. They increase in number, and their numerous canaliculi replace the 

 primitive pore canals. 



3. The cancelli break down, owing largely to the increasing number of vascu- 

 lar channels (Haversian canals) and their more uniform distribution throughout 

 the various layers, this process gradually producing a condition similar to that 

 in the typical dermal bones of vertebrates. 



4. The armor breaks up into separate plates or "bones" of various sizes, 

 which may or may not be movably articulated, and which may bear some defi- 

 nite relation to the underlying organs, such as the primitive subdivisions of the 

 head, or the arrangement of segmental muscles, appendages, or jaws; or they 



