482 Mr. D. W. Devanesen. Development of the Calcareous 



root-end of an adult tooth. From the nature of the case one should expect 

 the process in the adult to be similar to that in the young. Hence a 

 comparison between the two is inevitable, nay, compulsory. I am bound to 

 confess that my own researches do not confirm in their entirety the con- 

 clusions drawn by Giesbrecht, the last and must important of the investi- 

 gators uf the root-end of the adult urchin-tooth. 



According to his observations there are two sets of units for the urchin- 

 tooth, one for the wings or the lateral parts and the other for the carina. 

 The units of the former are Hat structures called " Scales" (Schuppen), which 

 are not homogeneous but are made of two lamella) separated by a narrow 

 interspace. It appears to me that, in this latter inference, Giesbrecht has 

 been misled by an artifact, and, likewise also in his other, mentioned in the 

 context, that his second set of units, " the prisms," have an axis- cylinder. 

 Each scale is undoubtedly a homogeneous structure of integral value. For 

 this reason, and also because it is more appropriate, I have, in the description 

 of these plates, retained the use of the term " lamella," and dropped out the 

 word " scale." His account of the manner of growth of each " scale " is not 

 in perfect accord with mine. He says : " Meanwhile, cpiite like the shell of a 

 mussel, lime is deposited in layers round one ' initial point ' (the ' calcareous 

 particle ' in my description),* though not uniformly in a circle but on one 

 side only, so that the initial point always remains at one edge of the plate 

 like the umbo of* a mussel shell." Though this description suggests a 

 peripheral growth, it differs in two respects : (1) the growth in the pre-cone- 

 formation period is by accretion in a straight line along the base of the 

 triangular plate (text-fig. 1); and (2) the concentric rows of stripes he 

 mentions in the context are non-existent in the lamella. Further, Giesbrecht 

 describes the lamellae of one row as alternating with those of the other, and 

 has entirely missed out the formation of the cones. 



To me, it would appear, that his second set of units — what he calls 

 " prisms," but, in reality, long needle-shaped structures — is of the nature of 

 secondary calcification. In the young imago, the carina is found to arise 

 solely by the fusion of inwardly directed flanges (text-fig. 1) of the incipient 

 cones, as has been already described in the section on teeth. Giesbrecht 

 himself speaks of a certaiu part of the lamella taking part in the formation 

 of the carina. I cannot, therefore, agree to his giving the " prisms " a 

 morphological value equal to that of the lamella;. 



On account of the nature of the methods he employed, such as dissecting the 

 root-end with a mounted-needle and making ground-sections (Diinnschliffen) 

 of the same, I do not know how far one can rely on these doubtful details 



* The parenthesis is mine. 



