OBSERVATIONS ON THE TERATASPIS GRANDIS, Hall, 

 The Laro^est Known Trilobite. 



By J. M. Clakke. 



Communicated to the State Geologist December, 1890. 



Trilobites of great size have been reported from various forma- 

 tions. With rare exceptions, however, these relics are but fragments 

 of the test, leaving to the imagination the restoration of the original 

 proportions of the animal, and without an earnest mental effort one 

 is apt to leave the contemplation of the large fragment with no ade- 

 quate conception of the imposing lineaments of its owner. Indica- 

 tions of these gigantic forms occur in all the grand faunas of the 

 Palaeozoic, with the exception of the Carboniferous where diminution 

 in numbers was accompaniecl b}^ diminution in size, or, in other words, 

 by the prevalence of genera in which great size was never attained. 



Almost with the earliest known appearance of the Trilobites the 

 genus Paradoxedes attained magnificent proportions. Paradoxides 

 Harlam, the well-known species of the Braintree argillites, must have 

 grown to a length of 18 inches. Angelin has figured an entire speci- 

 men of P. Tessini 12 inches in length, and Barrande a fragment of an 

 individual of P. imperialis which must have had about the same size. 

 Mr. G. F. Matthew has described a nearly entire individual of an 

 immense P. regina from the St. John beds, 15 inches long and 12 

 inches across the base of the cej^halon, and it is claimed, with 

 undoubted accuracy, that this is the largest undismembered specimen 

 of a trilobite found in any country. 



In the second faunas great Asaphids were not uncommon. As 

 early as 1839 Dr. John Locke described in the report of the Geological 

 Survey of Ohio, a portion of an immense pygidium to which he gave 

 the name Isotelus maximus. In 1843 Dr. Locke figured an entire indi- 

 vidual of what he considered the same species, changing the name, 

 however, to Isotelus megistus. This specimen measured nine and three- 

 quarters inches in length. The figure was accompanied by outlines 

 of two large pygidia, the greater of which was that referred to in 1839, 

 which, the author says, coincided " with the end of an ellipse 22 inches 



