NO. 2051. VARIATIONS OF THAMN0PHI8 ORDINOIDES—THOBIPSON. 355 



This specimen varies from the normal in four additional characters, 

 the shields involved being the postoculars, the posterior temporals, 

 the supralabials, and the urosteges. This is the largest number of 

 variations found in a smgle specimen. It also possesses the lowest 

 number of scale" rows, 13, that has been recorded in this species. 

 The reduction is brought about by the suppression of the scale row 

 adjoining the median. Doctor Ruthven has demonstrated that the 

 normal sequence of suppression in Thamnoiiliis, m terms of the max- 

 imum number of rows, 23, for the genus is the V, VI, IV, and VII 

 rows.^ These are adjoining rows, whereas the reduction in this speci- 

 men from 15 to 13 rows was brought about not by the suppression 

 of an adjoining row, but by that of one several series away, the one 

 next to the median. Some 10 species of DipsadomorphinsD have been 

 examined and where the scale formula was 21-19-17, 19-17-15, or 

 17-15-13 the sequence of suppression was first a lateral row, the IV, 

 and then the row adjoining the median; or the sequence was reversed, 

 and the row adjoining the median was the first to be suppressed 

 followed by the row on the flank. 



BILATERALLY ASYMMETRICAL. 



No. 50260, male, ventrals 163. Anteriorly there are 19 rows. On 

 the right side the V row is absent; on the left side it is very short, 

 as it begins at the level of the thirty-ninth gastrostege and is sup- 

 pressed at the forty-second. Over this segment of the body there 

 are 20 rows, and posterior to it the count of 19 is resumed. The IV 

 row is suppressed on the right at the eighty-sixth and on the left 

 at the eighty-fifth gastrostege, leaving 17 rows, which are continued 

 throughout. It will be noted that this series of three scales in the 

 V row on the left side is an mtermediate condition between the 

 normal 19-21-19-17 type and the reduced 19-17 type. 



No. 50313, male, ventrals 157. On the neck the IV row is sup- 

 pressed on the right at the fifth and on the left side at the sixth 

 ventral. Anteriorly there are 19 rows; on the right side the V row 

 is absent, on the left it beguis at the thirty-fifth, and is suppressed 

 at the forty-first ventral; over this short segment of the body 

 there are 20 rows. Posteriorly the IV row is suppressed on the 

 right at the eighty-third and on the left at the eighty-second ventral, 

 leaving 17 rows which are continuous throughout. It will be noted 

 that this specimen closely approaches the 19-17 type except for the 

 short series of scales in the V row on the left side, which extends over 

 the space of but 6 ventrals. 



OTHER VARIATIONS. 



Besides the variations in the scale formulae the specimens vary in 

 eight additional dermal characters. The sliields involved are the pre- 

 oculars, the postoculars, the anterior temporals, the posterior tempo- 

 rals, the supralabials, the infralabials, the anal, and the urosteges. 



1 Bull. 61, U. S. Nat. Mus., p. 17. 



