Observations on the Terataspis grandis, Hall, the largest known trilobite

OBSERVATIONS ON THE TERATASPIS GRANDIS, Hall,
The Largest Known Trilobite.
By J. M. Clarke.
Communicated to the State Geologist December, 1890.
Trilobites of great size have been reported from various formations. 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 adequate conception of the imposing lineaments of its owner. Indications of these gigantic forms occur in all the grand faunas of the Palæozoic, with the exception of the Carboniferous where diminution in numbers was accompanied by 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 Paradoxides attained magnificent proportions. Paradoxides Harlani , the well-known species of the Braintree agilities, must have grown to a length of 18 inches. Angelin has figured an entire specimen 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 cephalon, 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 individual 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 long and 12 inches broad. This is an evidently much compressed fragment, measuring seven inches in its greatest transverse diameter, and assuming this as the greatest diameter of the pygidium and restoring the length from the proportions of the animal as there given, the original length of its owner would have been about 13 inches. The plate is incomplete on its anterior portion, and it is probable that the error in this estimate due to the exaggeration of size from compression of the shield, is compensated by the loss of diameter from imperfect retention. This great pygidium, with other large fragments of the same species, were used as a basis for a well-known restoration in plaster to be found in some of the older museums of this country. Angelin has given a restoration of Megalaspis heros 14 inches in length and Brögger estimated the original length of Megalaspis acuticauda to be fully 16 inches. Barrande figured an entire Asaphus nobilis from Etage D which measures 10½ inches.