In previous papers by the writer on the structure and appendages of Triarthrus, no attempt has been made to describe or illustrate the character of the ventral integument, especially in the sternal or axial region. The specimens hitherto described were prepared to show details of the appendages, and though portions of the ventral membrane were exposed in many individuals, the subject was not considered of sufficient moment to warrant a distinct study, particularly as no characters were observed in the cuticle that had not been previously seen in more or less perfection by Walcott in the genera Ceraurus and Calymene. A recent discovery by Jaekel, however, necessitates the separate consideration of this structure. This necessity arises from the fact that a positive addition to the knowledge of the trilobite anatomy may be deduced, although, as will be shown, Jaekel was apparently entirely misled in his interpretation of the nature of his discovery.