Fossils of the Utica Slate. 



31 



tranverse, the dorsal furrows are less strongly defined, the occipital 

 segment is transverse and of the same width all across, the occipital 

 spine being proportionately smaller; changes of equal extent charac- 

 terize the thorax and pygidium. A comparison of the figures on PI. 

 II, will enable the palaeontologist to judge of these differences in 

 the two extreme forms of development; between these the changes 

 from one degree to another are so gradual that it is difficult to 

 note where one form begins or the other disappears. 



There is a variation in the pygidium of the last four degrees of 

 development, that is not at all constant, the last ring in the axis of 

 the pygidium merges into the margin, so that there appears to be 

 but four rings in the axis; as they are usually developed in the same 

 space, it is not noticed in the table, and five, the usual number in the 

 axis of the pygidium, is given. 



Table or Developmext. 



The first line indicates the number of thoracic segments and the 

 degree of development; the second, the entire length of the aver- 

 age individual of each degree; the succeeding three, the length of 

 the head, thorax and pygidium. As it is difficult to compare the 

 relative proportions of different parts of individuals of the different 

 degrees when expressed in fractions of the millimetre the succeeding 

 three divisions are so arranged to enable this to be done readily; thev 

 are based on dividing the millimetre in sixteen equal parts. By them 

 any part of an individual can be compared with any part of an indi- 

 vidual of any other degree of development. With a sufficient 

 number of examples of each degree, a series might undoubtedlv be 

 selected showing a regular proportional development of all the parts 

 throughout the series; this table is based on the material we now 

 have in the collections mentioned in the table. 



