124 WILLIAM PATTEN. 



nature. (2) The small size of the trunk at the posterior opening 

 of the dorsal shield, and the presence of what appear to be dermal 

 plates within the thoracic buckler, suggest that the trunk extends 

 forwards within the buckler nearly to the head region, and that 

 it is attached to it only along the low ridge and to the two large 

 processes in the mid-dorsal line, to the posterior corner of the 

 posterior ventro-laterals, where there are two large scars and a 

 transverse ridge, and to the internal ridges near the junction of the 

 cephalic and post-cephalic portions of the buckler. (3) The fact 

 that the greater part of the buckler is usually filled with the same 

 kind of matrix as that on the outside, while the part containing 

 the finer matrix, and indicating the presence of organic material, 

 forms a comparatively narrow band. (4) If the gills are located 

 in this large chamber, the water must have passed out of the 

 buckler at the posterior end, on either side of the trunk. Such a 

 condition would harmonize with the fact that the cloaca is situated 

 so far forward, for without such a current of water the excreta 

 could not be easily discharged. 



In conclusion, we may add that if the gills are located in the 

 thoracic portion of the buckler, it becomes extremely probable 

 that Bothriolepis possessed an atrial chamber similar to that of 

 Amphioxiis. The formation of such a chamber could have been 

 brought about by the extension of the margins of the cephalic 

 shield of such a form as Cephalaspis and their union on the ven- 

 tral side, thus enclosing almost the whole ventral surface of the 

 trunk from the anus nearly to the mouth, including the marginal 

 appendages. The latter would then occupy about the same 

 relative position as the lamellae of Bothriolepis. 



There has evidently been a forward extension of the ventral 

 shield of Bothriolepis so as to partly cover the plates imbedded 

 in the membrane of the oral region. The whole of this region, 

 back to the ridge on the inner surface of the anterior ventro- 

 laterals, with its four rows of plates, corresponds to the uncovered 

 oral region of Tremataspis, with its four rows of small plates sur- 

 rounding the mouth. 



HANOVER, N. H., 

 June 21. 



