MEMOIR OF CHARLES EMERSON BEECHER 545 



for this early stage in other genera, but as no other young shells were 

 at hand, he resorted to a study of the beaks in well-preserved examples 

 of mature shells. In the spring of 1891 he announced that he had seen 

 the initial shell in 15 families representing 40 genera. 



A study of the stages of growth in many brachiopods, from the Cam- 

 brian to the living forms, enabled Beecher to show that the old classifica- 

 tions were not expressive of genetic relationship. He demonstrated that 

 on the basis of types of pedicle openings all brachiopods are naturally 

 grouped into four orders, of which two are without, and two possess hinge 

 teeth. The most primitive order (Lingula, etcetera) he named Atremata, 

 and this gave rise directly to the Telotremata (Rhynchonella, Terebratula, 

 etcetera). The Neotremata (Crania, Discina, etcetera) also originated in 

 the Atremata, and from the former descended the Protremata (Stro- 

 phomena, Productus, etcetera). 



In 1893 there was discovered in the Utica formation near Rome, New 

 York, a thin band in which nearly all the trilobites occur as pseudo- 

 morphs in iron pyrite and retain antenna? and legs. Trilobites with legs 

 had been known before in two specimens and in four genera. Walcott 

 determined the presence of legs by slicing enrolled individuals. Antennae, 

 however, had not been clearly made out until 1893, when their presence 

 was announced by Matthew in the August number of the American 

 Journal of Science. This discovery was of great value and promised 

 much toward a better understanding of the ventral anatomy of trilobites 

 and their systematic position among the Crustacea. Beecher was thus led 

 to visit the locality in 1893, when he took out several tons of shale ; since 

 then he has published fifteen papers on trilobites. Of these, three are 

 devoted to the larval stages, seven to the ventral anatomy, and five to the 

 classification and systematic position of these forms. 



Beecher showed that in Triarthrus the entire series of thoracic legs are 

 biramous, one being setae-bearing and used for swimming and the other 

 without setae and used for crawling. The limbs of the pygidium overlap 

 each other, are much crowded, and are adapted for swimming or guiding 

 the animal, although they may also have served as egg-carriers. The 

 head has five pairs of appendages, four pairs of which are biramous and 

 closely resemble the thoracic legs. 



He also observed that in the first or unsegmented stage of the most 

 primitive trilobites there are neither dorsal free cheeks nor eyes, but that 

 in some of the later forms both the eyes and free cheeks have migrated 

 to the anterior margin or may even have progressed a little posteriorly 

 down the dorsal side of the first or unsegmented stage. This led him to 

 undertake a study of all trilobite genera, more than two hundred in 

 number, and it was seen that these could be arranged in three groups or 



