166 PROCEEDINGS OF THE TfATlOifAL MUSEUM. vol. 41 



structural feature the species has derived its specific name. From 

 this horn there originate two sets of ridges, one running anteriorly 

 and one dorsally. Taking their origin well toward the tip, one on the 

 upper or dorsal margin of the horn and another on the lower or 

 ventral margin, there run forward two ridges of the first set. The 

 upper one, the superior spinal ridge, continuing across the base of the 

 rostrum merges with other ridges which have a downward course and 

 disappears at the tip of the rostrum. The lower one, the inferior spi- 

 nal ridge, also passes forward; it diminishes in height and merges 

 with the general level of the honeycomb cells at the upper end of the 

 anterior border of the valve. The space thus bounded by the supe- 

 rior and inferior spinal ridges of each side, the lateral spinal cleft, is 

 free of structures. Similarly there run dorsally from horn to horn 

 two ridges of the second set, the anterior and the posterior inter- 

 spinal ridges, taking their origins halfway to the tip of the horn, 

 bounding the interspinal cleft, which runs from horn to horn over the 

 back. This cleft also is free from structures and divides the dorsum 

 into anterior and posterior portions. In the middle of the space 

 anteriorly is a clearly defined area, to be called the anterior dorsal 

 area, bounded laterally by high dorsal ridges which converge ante- 

 riorly with sharp lateral and apical angles. This area is divided 

 into large cells. The arrangement of these cells usually conforms 

 to the following plan: occupying the anterior angle is a single cell, 

 then four transverse rows of cells with two and three cells in each 

 row alternating; behind the last of these rows is a large pentagonal 

 cell, and between this cell and the anterior interspinal ridge is a row 

 of several smaller cells. 



From the antero-lateral angles of the anterior dorsal area just dis- 

 cussed there runs laterally and downward on each side a high ridge, 

 the cervical ridge, to the superior spinal ridge. The lateral space 

 bounded by the two ridges last mentioned, the anterior dorsal area, 

 and the lateral parts of the anterior interspinal ridge is occupied by 

 deep cells; large rectangular cells adjoining the dorsal and cervical 

 ridges, but hexagonal and smaller cells toward the other boundaries. 



From the anterior angle of the anterior dorsal area there runs for- 

 ward in the median line a short occipital ridge which divides right 

 and left into the facial ridges. The facial ridge on each side con- 

 tinues parallel to the cervical ridge and meets the superior spinal 

 ridge at the base of the rostrum, where their identity merges. Be- 

 tween the cervical ridge, on the one hand, and the occipital and facial 

 ridges on the other, there pass parallel partitions dividing the space 

 into long, deep, rectangular cells, some seven or eight in number. 

 Included between the two facial ridges is the facial area. In the 

 center of this is a small deep cell, the ocular cell, immediately behind 

 which is the compound eye. From the ocular cell and from small 



