504 HERRICK E. WILSON 



between plates and not between their formative eell groups. The 

 intrasntural deposit is formed through the activity of the ameboid, 

 calciferous cells, and the firmness with which the plates are united 

 depends upon the amount of stereom deposited. In youth the 

 deposit is slight, the plates are easily separated, and the sutures 

 are usually discernible as external or sometimes as internal grooves. 

 In age. however, much variation exists, for the extent of deposition 

 depends upon the stage of development reached by the group and 

 upon the vitality of the individual. In some cases immature (in- 

 complete^ anchylosis is apparently an adult characteristic and the 

 plates are easily separated. 1 In other cases the deposit is as strong 

 as the stereom of the plates, and fracturing results as readily in 

 the plates as in the old suture plane. Again, in cases where firm 

 union is the rule, as in the Camerata. lowered vitality, or other 

 physiological disturbance, sometimes results in the partial or total 

 inhibition of anchylosis. Such abnormalities, or reversions, are of 

 great value in determining the position of sutures otherwise un- 

 traceable, and will be more fully considered under the topic of 

 delayed anchylosis. 



From the foregoing definition of anchylosis the conclusion is 

 drawn that any suture or group of sutures appearing in the primitive 

 basal cup may be lost through anchylosis, and the following dis- 

 cussion will show the actual and possible combinations due to simple 

 anchylosis alone. 



In order to facilitate this description and the tabulation of the 

 variations to be cosidered. the convention of lettering the basal 

 plates from the posterior to the right-posterior, in an anticlockwise 

 direction, has been adopted, the letters separated by dashes 

 making up the basal formula. The letters from a to c denote 

 the plates and the dashes the intervening sutures; a dash over a 

 letter shows trunkation of that plate, while the absence of a 

 dash between the letters denotes the absence of the suture between 

 those plates. Thus, a — ft — c — d — e — is the formula for the simple. 

 pentagonal base while a — b — cd — e — shows a hexagonal base with 

 the posterior basal trunkated and the anterior pair of basals 

 united. 



1 Ref. i, p. 29; ref. 10. pp. 36, 37. 



