Fossils of the Utica Slate. 



29 



mens the third pair of glabella furrows ' and the peculiar short 

 transverse furrows. Plate ii, figure 14. 



Periods of Development. 



Professor J. Barrande divides the development of Sao hirsuta 

 into two periods. The first includes all the degrees from the first, 

 when there is no thoracic segment or pygidium defined, to the nine- 

 teenth, when there are seventeen liberated segments in the thorax 

 and two in the pygidium. 



The second period extends from the last (nineteenth) to the com- 

 plete development in size and ornamentation. 



It is difiicult to establish two periods in the development of 

 Triarthrus Becki; there is no change in the ornamentation, as it is 

 the same in individuals of from thirteen to sixteen thoracic segments. 

 There is a certain increase in size after the sixteenth segment is 

 liberated in the thorax, but not of sufiicient importance to indicate a 

 period of development. If any change is to be noted, it is, that, 

 after the development of the twelfth segment, individuals having 

 the same number of thoracic segments vary very much in size, 

 some even being smaller than those having a lesser number of seg- 

 ments; this period of development is a marked one in the history of 

 this trilobite; as shown by the table following an individual of thirteen 

 thoracic segments is larger than one having sixteen. Again we find 

 that an individual of thirteen thoracic segments is more than three 

 times as large as one with fourteen, one being twenty-four and the 

 other seven millimetres in length; that the largest with fourteen seg- 

 ments, thirty millimetres in length, is nearly double the smallest with 

 sixteen segments, and that the adult individual of sixteen thoracic 

 segments is fifty -three millimetres in length. Minor variations have 

 been noticed in individuals having less than thirteen thoracic segments 

 but in no case has the size of the one having the lesser number of 

 segments exceeded the next in the series of development. It is not 

 until the twelfth degree of development is passed that this strange 

 anomaly occurs. Until a much greater quantity of material is 

 studied we should not be willing to make a division and call this 

 a second period of development, as such a great degree of variation 

 occurs in this trilobite that our having but three examples of the 

 twelfth degree of development forbids such a separation except, pro- 

 visionally, for reference. 



^ Seen in individuals of ten millimetres, and upwards, in length. 



