CEPHALIC SUTURES AND THEIR BEARING ON CURRENT CLASSIFICATIONS OF TRILOBITES

Summary 1. The cephalic sutures are defined; their distribution and interrelationships discussed. Few facts are available concerning the evolutionary movements of these, but various theoretical deductions have been made. 2. Opinion on the supposed function of these sutures is summarised and it is concluded that the sutures probably did not exist solely for the purpose of ecdysis. 3. Consideration is given to the historical aspect of trilobite classification into three orders where the ordinal character is the course taken by some of the cephalic sutures. This classification has been adversely criticised; attention is called to the fallacy of assuming that the chosen cephalic sutures were: ( a ) relics of primary segmentation, ( b ) homologous in all groups, ( c ) in themselves alone of phylogenetic significance. 4. Three orders have been erected in turn to accommodate, according to their founders, the most primitive trilobites; none contains a single family in common with another of the orders. Pompeckj's and Swinnerton's objections to Beecher's order Hypoparia are upheld, as are Warburg's and Rud. Richter's objections to Swinnerton's order Protoparia. 5. The order Mesonacida (as defined by Poulsen) has a unique position; Poulsen regarded it as the most primitive group of known trilobites; Raw, as the most specialised group at least as far as the cephalic “segmental” spines and sutural evolution is concerned. Disagreement is expressed with this opinion of Raw's, essentially because of a more probable interpretation of the particular segmental origin of one series of spines homologised by Raw, and because there are doubts whether the remaining two spine pairs are necessarily of metameric (segmental) origin. 6. Though absolute proof is as yet lacking, Swinnerton and Poulsen are thought to have justifiably stated that the absence of true facial sutures in Mesonacidae is primary; some of the supposed primitive features of the family (or order using Poulsen's restricted sense) are discussed. 7. The bearing of recent ontogenetic work on the relationship of Proparia and Opisthoparia suggests that the proparian condition may be regarded as arrested development, and therefore previous failure to recognise this has resulted in the establishment of a group here held to be polyphyletic. 8. A satisfactory classification of the group might be evolved, as sufficient reliable knowledge accumulates, by combining allied families into superfamilies. Two attempts at this form of classification are discussed and summarised; these however are based on the primary ordinal value of the supposed (adult) static condition of part of one of the cephalic sutures, and can only be regarded as provisional.